Import the portrait to the same folder the model is in, name the portrait to (The model you want the portrait to show up for)_Portrait.mdx
Your model is in say, war3imported.
Example: I want the Footman to have the Peasant's portrait. So, I use a program (I use MPQEdit) to extract the Peasant's portrait from the MPQs, then import it to my map.
I name the file,
Units\Human\Footman\Footman_Portrait.mdx
You can do this with ANY model, as long as it's the same name as the model you want the portrait for with _Portrait at the end of it. It also takes normal portrait rules, in that it HAS to be in the same folder of the model you're trying to get the portrait to show up for.