I'm looking for software that can capture and edit packets (before they "reach" my computer). Like Fiddler, but for another ports and protocols.
WireShark is the obvious, industry standard, tool to record and decode packets for many protocols. I am unaware of any equal.
While I have never used it to modify the packets in transit, Google indicates that it can be done. Search for
modify packets using wireshark, watch this video and read the other search results.
If I were you, I would look no further. If it has any possible downside, it might be that it is too powerful and full-featured ;-)