Hm, I think it would be easier to change the Triarii into your new unit, but use an edited cohort model.

To edit the model, you need 3ds max. If you don't have this expensive program, you can get GMax for free, which I understand can open, but not save .CAS files. If you use this, I'm sure someone of the forums will convert your model into .CAS for you.

You need to get Verc's .CAS extractor, find it in the tools thread in the guides forum. Then you just open the triarii, copy the spear, open the cohort, add the spear, and save as a new model. You then need to make this the model for your unit in descr_models_battle.txt.

That might seem a bit confusing, coz I assume you know something about modding units. If not, I'll go through it in a bit more detail
Hope thats more helpfull than last time.