I've trying out some little video recording tools, though I'm not really sure what are the best free software out there to do this sort of thing.
The one I've got now, CamStudio, is a bit sluggish on my system.
So open for suggestions on that and I'll create some new tutorials, both written and vids, which I'll upload to my youtube channel.
To quickly anwser your question:
Extract both the MDL and MDX file for your chosen model.
Run MDLops, pick the game the model is from. Then hit the Read and Write button.
This will create an ASCII version of your model.
Then start up Gmax, with NWmax installed.
In the NWmax, panel, mostly this located on your left side of the interface.
Look for the tab: MDL loading.
Hit the button BROWSE.
Track down that ASCII file!
Then click on IMPORT button.
Hold on thight; depending what model you're importing it make take a few seconds to a minute
Voila, you should now have your model in Gmax.
You can add all sorts of modifiers and mess around with the thing.
Note that around your model is this bright green square!
That's the Aurorabase; you can select that.
On your rightside of the interface, you can find the options on what the Aurorabase does.
One of it's main features is, yes; look for it! Exporting!
You can setup a path of your own choosing to where you want this new model to be exported.
Again it will be an ASCII file. Which means you'll need MDLops again to "bake" it to a proper MDL and MDX format the game uses.
Very quick and dirty, but that's the gist of it