Different source udp port per user

My Asterisk server has several users configured. There is also a SIP trunk configured to an ITSP (no registration).

The ITSP authenticates and bills based on the source IP:port of my Asterisk server. I want the ITSP to be able to bill each customer individually.

Is it possible for me to have a different source udp port for each customer on the Asterisk, so that the ITSP can differentiate between my customers?

Thank you
Richard