The Secrets of the Third Reich forum has a free PDF download with a Partisan Mortar unit that can be used with the Partisan force from the upcoming Doomsday expansion for SoTR.
From their website:
Attached is a new official SOTR unit which may be included in any force constructed with the Partisan rules in DoomsDay. Also included for handy reference is an updated force organization chart for Partisans which includes this new unit. Enjoy.