is it possible to

[Ignopotens]

I dunno about actually modding it into the game file, but you could just attack Syracus with 1 unit and use auto_win attacker (or auto_win defender if they sally) and then give it to the KH via Force Diplomacy

[Aemilius Paulus]

Modding desc_strat is one way.

[Maion Maroneios]

It is doable though it requires quite some changes. Even a minor typo or misplacement of a line or space will lead to a CTD. As for them being allied with the Koinon, that's completely unhistorical. They might have been Hellenes as well, but Syrakousai was an independent polis that acted on her own. By the way, in RT'R', the Greek Cities have southern Hellas, Epeiros as well as Syrakousai. I mean, how far in terms of (un)'Realism' can it go?

Maion