Use pre-existing directory for assets

The project that I'm working on already has a pretty extensive file structure built up, with our files organized hierarchically and all that good stuff. I'd like to bring all of this into a Unity project, while leaving the files in place. So far it seems like I can make a new project and import the root folder, but then it just copies all of the files to its own location.

Is there any way to make a Unity project that uses a pre-existing directory as its Assets folder?

No, because of the associated metadata, but you can select the assets and export them as a package.