The most simple thing you can do is attach the helmet to the head bone if it isn't already (OpenBRF press Attach to Head whatever).
Then make it a sub-item of the part which makes your class unique and after you're done delete the actual mesh from the helmet.
basically rename like british_33rd_uniform.1
Or however it is called.
Also make sure that EVERY unit using this hat now has atleast the normal hat attached to their uniform.
Be careful though because if the uniform or whatever part you attach your hat/helmet isn't unique you will see these hats combined with others.