You could copy the Seleucid names, but you need to make sure there are no duplicates in the list (in descr_names.txt). Tedious business. Don't remove any without checking descr_strat.txt in the campaign you are working on, as you may get a CTD.
As for the unit you mentioned, yes, duplicate the texture and sprite setting in DMB from Seleucid for 'numidia'. Then add numidia to the ownership of the unit in EDU. Then put the unit in the 'greek_order' barracks which only numidia (Pergamon) uses.
T make it all pretty and such, copy the unit cards (small and large) into the numidia folders from the seleucid folders.
Alternate history campaigns can be a lot of fun, actually. I've been working off and on (mostly off, lol) on an alternate history Roman campaign for RS2. In this particular scenario, the Romans lost the 2nd Punic War. Hannibal sacked Rome and left. Capua formed the 'Capuan Republic' and controls southern Italy. Carthage is happy to let the much weakened Rome duke it out with a more powerful Capuan state, so has went about it's business building an economic Empire. I think the scenario could be fun, because the lack of Roman influence has caused incredibly powerful Celtic states to arise in Europe...indeed, the whole face of the map is different because Rome had no power to interfere. Macedon has emerged as a powerful Empire bent on reclaiming what was Alexander's. Parthia has beaten the Seleucid's down to a few coastal regions. Macedon is at war with what remains of the Ptolemies, and has taken Alexandria. The year is around 30BC, and the Capuans have just taken Umbria from the Romans...defeating a Roman general who was betrayed by the Senate, and deceived into believing the Senate would provide support for the war in Umbria. But the Senate is afraid and more determined to pacify the Capuans, so they left their General hanging, and he lost. His name is Julius Ceasar....and he's pissed.