I recently read through a tutorial to re-skin models in Attila from Rome 2. Now this person who posted the tutorial also posted a Pack file with the same type of format that Attila uses and it seems fairly easy to do
The link is here
And this is the how to of it
1. Open PFM and create new file
2. Add the necessary models, for vanilla Rome 2 units this is done by:files -> add -> from pack
a new screen will open, go to your Rome 2 Data folder and select models_rome2.pack
a new screen will open, expand models_rome2.pack (+ sign), expand variantmeshes, expand _variantmodels, right click man (good job you've just added the models

)
expand variantmeshdefinitions and double click the model/unit you want to use
click ok
3. Add the unit whoes model/skin you want to chance, for vanilla Attila units this is done by:files -> add -> from pack
a new screen will open, go to your Attila Data folder and select models.pack
a new screen will open, expand models.pack (+ sign), expand variantmeshdefinitions and double click the model/unit you want to change (huzay we've got all the files we need

)
Now the editing begins
4. Right click the Rome II unit under variantleshdefinitions and click open -> as text
5. Copy the whole thing on the right hand side
6. Right click the Attila unit you want to change and click open -> as text
7. select everything and paste it over
8. Save it
9. Congragulations you just changed the unit, go and enjoy 
Now the first times this can all be very confusing and dificult, but it gets easier the more times you try.
If you want to change other units, I would say you should try it, if it don't work or you hit some hard spots I (and a lot of other members I think) are willing to give a helping hand
My question comes into I am hoping that this thread can turn into more of how to really break up these areas of a model so we can swap and create what we want as much as possible.
I tried to take some really great skins I saw as a mod and do this and it did not work so I am hoping to find out how to get the models from these mod packs and make them work for Attila as well as there are so many great addons out there to be ported over or just added to your personal game.