[Serusers] Adding headers to the "200 OK" response

Is it possible to manipulate the "200 OK" response to a REGISTER method? I need to add a header field to it, and as far as I can tell, there really is no way to do this. Why do I want to do this? Well, Snom foneslook for the presence of a "P-NAT-Refresh: <yyyseconds>" header in the "200 OK" response to their REGISTER methods. If the header is present, the fones start sending out "keep-alive" packets every yyy seconds, thus keeping NAT bindings open Why dont I just use "natping_interval" in mediaproxy? Because there are a whole bunch of routers (linksys firewall/routers, dlink ones, etc.) which only keep the binding open when there is a packet sourced from the internal network. Cheers