UDPSZ is a nice tool for crafting custom udp packets (
http://aluigi.org/testz/udpsz.zip) It allows you to spoof the source ip/port, and send your data in various ways. However, I was a little confused with exactly what you're trying to accomplish so I'm not sure if this is a good solution.
Hping3 is also a very nice tool for crafting custom packets of many types --
http://www.hping.org/
Both tools run best on linux since Windows limits a lot of what you can do with your packets.