Would it be possible (perhaps via mod) to "tag them in", so to speak, by touching them with one of my non-merc armies? For example, if I am in a war and have troops far from my borders, I'd like to be able to supplement them by hiring a force of local mercenaries, and it doesn't really make sense that they'd need to walk all the way to my borders then all they way back to where they started in order to fight. Needing to bring them home first just seems like an unnecessary holdover from the teleporting merc system that is in CK2 and Stellaris.Agathocles also had help from some of Carthage's Numidian "allies".
In game terms we would consider the Numidians a type of Carthaginian subject, but that is for a later diary.
That said, if one wants to hire mercenaries to use against Carthage in the system described in this DD then it is necessary to first bring them home to Rome. They start out exiled after all.
Also, if my enemy fails to pay his mercs and I hire them out from under him, would I need to send them to my borders, then bring them back to where the fighting is, before they can actually fight?