I would assume that it's an easy change/addition to a config file. If we can add new ship models, you shoudl be able to crate a new torpedo model.
I found this site talking about homing torpedo development from 1940-1946:
http://www.geocities.com/Pentagon/1592/ustorp4.htm
... and post-war "heavy weight" torpedoes:
http://www.geocities.com/Pentagon/1592/ustorp5.htm
It looks like you are after the Mk. 31, which is a Mk.18 equipped with passive homing.