Had this question come up in the forum, and in fact it's very easy to
do, have many instances of Seagull running on user tables from other
projects/software. Makes migrating a project easier, plus a pleasant
surprise for you boss to get detailed Seagull user handling from one
of his old crappy projects ;-)
Here's one way of doing it:
http://trac.seagullproject.org/wiki/Modules/User/IntegratingWithOtherProjects
The Seagull project is sponsored by Seagull Systems, see the range of products offered.