Individual units can be applied directly to your ww2art.sga file (although it becomes overwritten and can no longer patch properly! Make a back up...) or integrated into a skinpack specifically for EiR:R. This involves unpacking the externalskinpack.sga file using corsix's mod tools in a similar fashion to modifying your ww2art.sga file (which can also be done with the skin installer), changing out certain folder directories, and then repacking the file.
As I said it's somewhat complicated, and I really don't have time for a step-by-step at the moment - I'll get you one if you want it later when I have more time
