Well, if those assets are freely available to Unreal Engine they would be allowed for an Unreal Engine project (as of EPIC's licensing terms), *but* since you can't redistribute the assets themselves (only game build that make use of them) you would have to leave them out of your project's source upload (regardless what method you use to give access to the source). Depending on how they work and integrate into the project it might be difficult to clean up your project to make it safe for upload. So while technically ok, it's probably much easier to not use them.