Extending Ettercap is an alternative to designing and writing pcap2john program from scratch.
Ettercap's homepage is http://ettercap.sourceforge.net/
For general information about Ettercap, see http://en.wikipedia.org/wiki/Ettercap_(computing)
However, all development takes place on GitHub https://github.com/Ettercap/ettercap
Pull requests are handled by Emilio A. Escobar but other developers participate in the discussions too.