At least as far as the Seleukid case goes:
In the case of the Armenians: should be farily easy to mod in.
In case of the Sabaeans... not that easily done. You see the Sabeans are on their own texture and that's not the merc one; but the model is also used for the Eranshar archer guys (who occupy both romans_julii and merc slots)

Though I dunno, perhaps the Slave faction has it's own model, you can check that with the EDU. If so you can try to award the romans_julii a slot on the slave model also. But only if the slave's don't happen to recruit the merc Eranshar guys. And I would count on that to spoil your fun.