SendFlagTcpPacketToTcp(Lastpacket,Flag,fromip,toip,from port,toport);
It would also need SEQ and ACK. Basically you can write such a function using the existing API completely in user mode.
by the way,how to get the IFS HOOK like winpktilter(NDIS HOOK)?
Create an IFS filter driver. There is a static IFS filter sample in the IFS kit (or in the latest WDK) names sfilter and dynamic IFS filter driver authored by Mark Russinovich used in Filemon. The source code for the last one was floating over the Internet, not the latest version but still.