You can certainly link projects together, though not in a way I think you are asking for. The reference pane can hold a link to any type of file on your system, including other Scrivener projects.
Beyond that, there is nothing like what you are referring to, and the reason for that is simple: projects are meant to contain everything related to that project. They are supposed to have all of the background material, research, revisions, the manuscript itself, and anything else you might need. There are, in some cases, practical limitations due to gigabytes of research material (then a separate program like DEVONthink Pro is probably a good idea to augment Scrivener with), but excepting that, there should be no reason to chain project files together.
But you may have some very different use of Scrivener than most people. If you want, let us know what you are wanting this feature for, and we might be able to help you find a better solution that works all within the same project.
There are many tools for this sort of thing with the Mac. You could just use the Finder comment field and then turn on the comment column in the folder where you store your Scrivener projects. That would be the cheapest. There are also programs like Leap, which are kind of like “super finders” when it comes to things like meta-data. Not many programs have “managers” for their own files. Generally because the Finder and other tools already do this job perfectly well already, so why reinvent the wheel with a limited implementation? That is the reasoning, anyway.
Since most people use one project file to work on one project, this is not an odd thing. I have three projects open right now. One of them is for Scrivener documentation, the other is a personal diary, another is a work log, documenting the things I do. I have no reason to connect the three together! It is nice to have them all open at once though, I don’t have to keep closing one to open another.
The import feature is main there for unusual cases. I would be most people don’t use that on a regular basis. It’s meant to merge two projects together and (most often) abandon the odd one out. Its for when two discrete projects are realised to really be best handled in one larger project. After imported, there is no need to keep the old project around.