it is possible to capture packes from a specific port an re-route it to an other ip?
Can i change the dest. & src IP-Address in the IP-Header?
Which stuff has to be modified?
Yes this is possible and it is as simple as changing IP/TCP/UDP header and recalculating the checksums. Also if this is sort of redirect the reversed operation should be performed in reversed packets.
Internet Gateway sample http://www.ntkernel.com/w&p.php?id=31demonstrates the usage of WinpkFilter for implementing NAT solution.