>> Is it possible to create users using SQL? I need to create an extra user
>> from my database and so far it looks like I'll have to do it either manually
>> or through an application. I would much prefer to cerate user through a
>> database upgrade script, though.

It all depends on the way/tool you use to run the script. On Windows you can try
http://www.volny.cz/iprenosil/interbase/fsql.htm
and type "help service" to get short list of extra commands.
(the version on web is about one year old, I will put newer version soon).

Ivan