The WinpkFilter library you mentioned, is it free or do I have to purchase it? Is it the free downloadable ndisapi.dll ?
WinpkFilter is free for non-commercial use and available for download from here (http://www.ntkernel.com/w&p.php?id=7). Package includes drivers(ndisrd.sys/ndisrd.vxd), API DLL (ndisapi.dll) and several simple samples.
If you register WinpkFilter (any type of subscription starting from 95$) you also get access to the Internet Gateway source code (http://www.ntkernel.com/w&p.php?id=31) which implements NAT and gives a good clue to how to redirect packets.
Some code samples can be also found by looking through this forum. I can also C&P a redirect relative portion of code from NeT Firewall source, but it may appear a bit difficult to understand.