A portrait of a model only works if the modle came with a portrait. To make the portrait center on a different unit, just use the action Send Transmission from unit type instead of from unit, and select the unit you think is closest to the model imported.
with the thrallnowolf model, there is no portrait, so,, i opened the .mdx and extracted the regular thrall portrait from units\orc\thrall\thrall_portrait.mdx then i imported it and gave it the name wc3imported\thrallnowolf_portrait.mdx (corresponds with wc3imported\thrallnowolf.mdx) and it worked great only problem is, when the person made the model of him unmounted, they didnt give him his dreadlocks (has 2, more of ponytails) so its a bit dodgy...
so, if there is a model with no portrait, and a simmilar model already exists, try exporting a portrait model
i use a mac so i use mpq2k but with a pc i think u use warcraft 2 exporter (correct me if im wrong) but u can download it from this website by going to the "Tools" menu at the top of the screen