There's no possibility to add to the bundle plugin or theme which belongs to other vendors. There are only your products in the select box.

It would be great to provide the ability to add 3rd party vendors. We believe it will push marketplace purchases, vendors collaboration, and October CMS revenue forward, as well. Let me explain, please.
As we're from the Russian-speaking community, we know the local market and local customers' habits well. There's a huge monopolist in Russia called Bitrix. Let's check its popularity among web design agencies in the CMS rating by cmsmagazine.ru.

Not the last reason of a such popularity is the existence of ready-made websites based on 1C-Bitrix. There's a 3rd party vendor Aspro, who sells prepacked 1C-Bitrix bundles for a wide range of businesses (ecommerce, corporate websites). And such a product is very popular among the customers. It's much cheaper and faster to launch a new website, especially you have a limited budget. Customers ask developers to customize these bundles after a while. This a good alternative to SaaS, which became more popular day by day.
We sell two bundles as ready-made websites:
We'd love to include in such bundles 3rd party plugins to extend the functionality of the bundle. Such plugin vendors will get more purchases in this case. Besides, we receive a lot of requests from third-party vendors regarding their wish to sell their themes with Shopaholic plugins. Exactly what we do by ourselves.
So, we believe such functionality will boost the marketplace to the new high. This's a win-win-win proposal (developers, customers, October CMS).
I want to ask the community, how do you feel about that? Please, rate this comment with 👍️ or 👎️ and share your thoughts.
The list of important things that should have been discussed:
I'm not selling any product on the marketplace yet so my mind may not reflect the reality.
As I know: free product can be "added" to a bundle, just by require them in the plugin or theme. So the problem comes only for the paid products.
If so I see both good improvements and problems here:
This was my think about that stuff I don't fully understand yet.
As @RomainMazB said, you can already include free plugins through dependencies. Paid plugins not by the bundle author would be incredibly complex to implement support for because you would get into incredibly complex situations around revenue splitting of bundle sales, handling discounts on the bundle as a whole as well as any discounts that a third party author might have on their product.
Personally, I'm just not convinced its worth the added code & accounting complexity.
@RomainMazB, thx for your opinion. As for free plugins, you right they can be added as requirements to the theme and it should be working. But there's a huge bug I've found and described in here.
As for the problems you've mentioned. Thx for the points, I'll add it to the commend above.
@LukeTowers, I just want to remind you of this issue #4728.
@RomainMazB, and as for coupons. There's no possibility to apply a coupon to a bundle while you purchase it.
@RomainMazB, @LukeTowers, as for warranty. I've never purchased a bundle by myself, I just sell bundles. =)) As a merchant I see the purchase of a bundle like that

I mean, I see 7 purchases of paid plugins included in the "Sneakers Shop for Shopaholic" bundle. I don't have an invoice for some of our customers to check how the purchase looks for them. I'll get it tomorrow.
Anyway IMO bundle page can inform a customer that bundle includes products of a 3rd party vendor, so the warranty and support for that part of a bundle are handled by its developer. And the customer will get a separate invoice for each 3rd party product he/she has purchased.
The problem still stands when it comes to how to split the revenue with the third party author. If they're charging $50 for their plugin, and you're charging $20 for your bundle, how would they get paid? And even if you're charging $60 for your bundle, what, that means you're only getting $10 for everything you added to it?
It opens up a huge can of worms when it comes to how to split the revenue of the sales, not only from a coding perspective but also from an accounting perspective.
If they're charging $50 for their plugin, and you're charging $20 for your bundle
The price field should be restricted to the min value of the sum of the 3rd party bundles.
And even if you're charging $60 for your bundle, what, that means you're only getting $10 for everything you added to it?
Yep. It's the merchant's responsibility.
@LukeTowers, you should know how things work. I mean setting the price value for a bundle. Let me show to others.
To set the discount to a single product, you should you can set the discount percentage value for a coupon.

To set the discount to a bundle, you should decrease the price of some product included

In the Sneakers Shop for Shopaholic bundle, I've decreased the price for the theme included.

Ahaha! I just clicked to the Approved link and saw this modal

I've never used this functionality! Anyway, there can be a restriction to change 3rd party vendor price value. Or there can be an input to apply a personal coupon to discount the value.
Upd. Oh my, I used this modal od course, just forgot about the _Percentage value_ checkbox
Some thoughts.
The list of the bundle products including 3rd party plugin.

Edit bundle product modal without the ability to change 3rd party product price.

Edit bundle product modal with the ability to change 3rd party product price using a personal coupon.

Make plugin available to add to the 3rd party bundles.

Hmm, not bad for UX @lautsevich, there's still the complexity of actually implementing it and dealing with the accounting side of it. I'll pass it along to @daftspunk though.
Great, thx!
BTW, AFAIK this functionality has been planned for the first release of the bundles' functionality. We've discussed it with @alekseybobkov for more than one year ago.
Yeah, I talked to Sam and it's still in the plan for future functionality to be added. No promises on when it'll be added though, @alekseybobkov's a busy guy.
I know, I know. I've opened this issue to discuss the idea with the community first of all.
There's not much to discuss AFAIK, it's a planned feature, and as always when it comes to the marketplace there is no guaranteed timeline of when it will be completed. If anyone has any specific problems or suggestions they can think of then they're welcome to add them to the issue of course, but no guarantee they'll be implemented. Voting on it also won't speed it up 😉