ASP.net get max value from Profile field

I have a classic ASP website containing a users table with a ID_USER field (int, primary key, auto increment). The ID_USER value is used to track user's activity and is saved in other tables as part of the "Saved by","Saved date" logic.
Now, the website was updated and moved to ASP.NET, with the authentication rewritten using the ASP.NET membership and Profile providers. The old user's table was imported in the new structure. The ID_USER field became a Profile value in the new Membership/Profile logic.
All the other tables remained the same becouse of the compatibility between the two websites.

Question:
When creating new users I need to set the value for the ID_USER field too.
How can I do this? Can I somehow get the max value of the ID_USER profile field?
Thanx

via Chebli Mohamed