That special Portrait apply thing is only available for destructibles.
For others such as units and buildings, portrait models will automatically be applied when you put it in the same path as the main model.
For instance, if you have ManBearPig.mdx and ManBearPig_Portrait.mdx, the ManBearPig_Portrait.mdx must be in the same path as the ManBearPig.mdx. The ManBearPig_Portrait.mdx will the automatically apply on the unit that uses the ManBearPig.mdx when ingame.
Understood?