Importing Custom Models into RoC maps

I'm looking for someone with strong knowledge of RoC map editing (NOT Frozen Throne) to help me.

I want to import custom models into my RoC-only map. I remember back in the day that you could only overwrite existing models (i.e. replace Units\Critters\Hydralisk\ with a new set of model files) and not import a new path with new filenames (i.e. import a folder NewLurker\ with new files and reference that in the Object editor).

If it is possible, can you point me to a tutorial or post the method here? For example, I've tried dropping the Mc-Lord Vexorian Diablo Model into my map with an MPQ editor, keeping their directory structure intact (DiabloModel\Textures, etc.), and pointing a unit's model path to DiabloModel\DiabloModel.mdl, but no luck.

Thanks in advance for any guidance!