Customized SIP Header Fields

Hey I’m trying to include some proprietary information within the INVITE.

Is this possible? Does anyone know if I can add text within SIP header fields without affecting the normal SIP messaging?

For instance if I put something other than the normal methods in the ALLOW field. So along with the normal methods;INVITE, ACK, REFER etc etc if I added CiscoMAC1234 as a proprietary identifier would this be allowed? Or would I get an error?

Are there fields that are already set-up to do this type of thing?

Any help would be appreciated.