Sure there is. By chance.

Spoiler Alert, click show to read: 
Open export_descr_buildings.txt and search for the precursor level of the "cataphract" building. That one will have a weapon bonus of -1; change it to 0 and you'll have your cats. Dunno if it's save-game compatible though.


Btw: Kinsmen cavalry are a quite useful and relatively affordable heavy cavalry arm.