Add optional session code per tech

I would like a way to assign a code to each technician so that when they create a new session, their assigned number is the session number. But if the user has not been assigned a code, it defaults to the normal random 4 digit session code.

Example:
User       Session Code Assignment
Daniel     0
Jim         1
Alex

When Daniel clicks "Create +" the session is made with a session code of 0
When Alex clicks "Create +" the session is made with a system generated session code