Hi there, you can migrate assets to a different project using the migrate tool in unreal engine. There’s some documentation about the topic and provides step-by-step instructions so you can easily follow along:
https://docs.unrealengine.com/latest/INT/Engine/Content/Browser/UserGuide/Migrate/
I hope that helps! 
Edit: Oops, apparently I read your questions wrong. haha sorry about that. I don’t think I can give a definitive answer to your question. I don’t have a lot of experience trying to open projects of others.
The only similar experience I have with that is using the content examples and free example projects in the Marketplace. Usually after you download the free example projects on the Marketplace it automatically has a “Create Project” button beside it and you can just click on that, but when it comes to using third-party projects that aren’t from the Marketplace then I have zero knowledge.