The BackTrack connection of this post seems pretty tenuous to me... but since my answer will mention at least one tool available on BackTrack I will allow this.
What do you mean by "need to be layer4" exactly? From what I have read about imsniff (in the last 30 seconds) it listens as a daemon, and I see no evidence that it sends anything onto the network so it sounds 100% passive to me.
If you want to feed a pcap file into the program check to see if imsniff can read from a pcap instead of listening as a daemon (most programs based on the pcap libraries can do this), and if it cant use tcpreplay to resend the data in the pcap file to the program.
You also might want to check out xplico, which is available in the repositories.