How can I add a "create project" asset type from my vault to an existing project?

I don’t understand why some vault items only allow you to create a new project?
This seems to prevent me from combining multiple of these projects as they would each require that I create a new project?

Thanks for your assistance.

I still don’t understand why some Marketplace Vault items are “add to project” while others are strictly “create a project” but I have managed to integrated some of those ‘create a project’ assets in to my existing project.

I basically created the new project for each asset that required it, opened it’s path at the file system level in Windows Explorer, and finally copied the content in to the content directory of my project. This is the best work around I could come up with, if anyone has a better one, please let me know.