Start by going to Falkooxc tools , choose HWP unit builder , choose the drawing routine unit that fits the most (mind the turret), download the ez to build sprite sheet, change it , after changes upload it on unit builder again , generate sprite sheet (the one openxcom reads), see the code.
Now to the coding part , just need to change the armor sheet and drawing routine , use the unit armor you copy (in the HWP unit builder) as reference because of lofttemp and other things i might be missing
The Rest you should now better then me , like items and units for more customization .
It would be great if we can make soldier wear 2x2 armor but dunno if that´s possible , so i think it was to be a item or a living weapon unit
Btw i use photoshop for image !