Thanks for your question. PST importer 2010 looks for any file with a PST extension. It does this when you install the PST Agent application on the computer from where you want to import PST files. The PST Agent will tell the PST Central Service about the PST files that it has found. So in your case, you would install the PST Agent on your file server and it should detect all of the PST files that your users had copied to the file server.
PST Agent does not care whether or not the PST is actually attached to a user's Outlook profile.
Hope this helps!