How To Modify Minisip?

Hello,
I'm trying to connect an Usar Agent IMS and a User Agent Minisip.
To connect these different UA I need a SEG (Securuty Gateway) in the middle of these 2 UA. The problem is that I'd like to modify Minisip in order that when it receive an Invite from the User Agent IMS, the Minisip send to it an error 420 (Bad Extension).
But now the Minisip answer with a 180(Ring) message.
So I don't know how to modify the behaviour of Minisip,since don't exist a source code or similar.
I'd like to know how modify the Minisip as I wish.
Minisip work on Windows XP.

Thanks and sorry for my english.

Regards