If you're already using an ODBC database as the back-end storage mechanism for users and groups, then the 3rd party system (or a script you write that's triggered by this system) could directly insert a user into the database. The 3rd party system could also be responsible for removing it at the appropriate time or you could leverage Serv-U's built-in functionality to automatically expire/delete the account on a pre-defined date.
Serv-U also supports asking a 3rd party DLL to respond to events within the system (such as a login attempt). By writing a DLL that implements some pre-defined functions and uses some pre-defined objects/values, Serv-U will forward the login information to your DLL and allow it to respond appropriately. In this way, you would write a DLL with its own user storage mechanism or have it communicate with some other system to determine whether or not the login is valid.
However, the ODBC database approach is definitely the easiest approach.