Currently, it is not possible to migrate or copy a list of users and their concominant rights/assignments from an existing VSS database to another one, using either shipped VSS features or programmatically, using IVSS. However, our capable friends in Microsoft's Product Support Services (PSS) have developed a tool that can perform this task automatically. You can download this tool from http://www.korbyparnell.com/tools/vssems.msi.
If you are the designated VSS database administrator on your team, VSSEMS can help you do much more than copy a user list from one database to another. Check it out. This utility is officially unsupported by Microsoft (or me for that matter :-). However, if you experience any difficulties while using it, please feel free to let me know.