Hi, does anybody knows is it possible to change tcp/udp packet size inside the packet filter hook?

I'm trying to add my data to the end of tcp payload, I use m_append function and as I see the mbuf is really contains my
 data after appending, but the server receives the original sized package without my data.

Also I tried to allocate a separate mbuf with my additional data and then to concatenate it with original buffer use 
m_cat, but it also didn't work.

Is it even possible?