Do you mean like add an already existing unit to Poland roster

I don't know if it exists. I think you can transfer textures from Total War Empire to MK1212 and replace one of the units in the Polish army. I know that winged hussars are in the Total War Empire. Or u can make it again. I have no idea how modders add new units
edit or u can reskin some polish unit to hussars

You have to check to see what kind of cavalry units are like hussars and add them to the faction roster

where to check? in mod mk1212? I think there is no unit similar to the hussars