[Users] Adding headers to the CANCEL messages generated by openser

Hello openser users, I'm trying to do something with openser but I haven't been able to figure out how to do this. So I hope somebody on the list knows the answer: In my configuration script I use append_hf to add application data to the SIP messages, this works great until the moment where I do a parallel branch. The invites work just fine since I can do the append_hf in the branch_route. The problem arises when one of the branches is answered, at this point OpenSER will generate CANCEL messages for the other branches and I have no clue on how to add header fields with information about the original invite to those messages. So my question is: Does somebody know how you can add headers with data to the CANCEL messages generated by SER in a parallel branch? Thanks in advance. Kind regards, Ardjan Zwartjes.