When Kamailio receives the INVITE, it relays it with two Record-Route like this : 

 Record-Route:sip:IP1;ftag=xxxxxxxx;vst=xxxxxxx;vsf=xxxxxxx

 Record-Route:sip:IP2;ftag=xxxxxxxx;vst=xxxxxxx;vsf=xxxxxxx

What i want is : 

 Record-Route:sip:IP1;ftag=xxxxxxxx;vst=xxxxxxx;vsf=xxxxxxx;NEW_PARAMETER=10

 Record-Route:sip:IP2;ftag=xxxxxxxx;vst=xxxxxxx;vsf=xxxxxxx;NEW_PARAMETER=20

I hope this make sense now.

Thanks for your help