Convert to virtualenv an existing project by copying site-packages
No, you can't. Python packages often have filesystem paths written to various metadata files. Just take the time to go through the site-packages and install the things into a fresh virtualenv, then call pip freeze to get a serialized list that you can use going forward.
I have an old Django project that I want to convert to use virtualenv. If I could copy the current global Python packages to the new env, I think I'd be assured that I'd have the same environment and would save myself some time over creating a requirements file by hand. So, could I just copy the global site-packages contents into the env's site-packages directory?