Unless modding is considered illegal or something by Longbow games (please don't ban me if it is, since plenty of games allow for modding without propogating any illegal activities)
As a single player game, you're welcome to mess around with the it anyway you like, and I don't think we've ever had to ban anybody from the forums yet. As DustinRush says, the only real limits are the obvious ones: you can't sell your mods or pass around your registration info. However, if you want to post your modded files (as long as they're not breaking the registration system) here or elsewhere you're perfectly welcome to.
All of that said, unfortunately Hegemony Gold isn't very moddable. This isn't because we're opposed to the idea, we really just didn't have time to clean up our tools so they were good enough to pass around. However, there is a globals.xnt file in resources/data that contains all of the units stats and most in-game parameters, and all of the textures are available in resources/images if you want to change the appearance of anything (if you do we'd love to see it).
In the long run, we are really hoping to better support modding in Hegemony Rome and I'd be interested in hearing what features you'd be interested in modding.
Nice to hear that. Regarding what I would like to mod, for now it would be just tinkering with appearances and stats of the units, buildings and resources to adjust them more to my liking (not that the current game is not fine; I just believe there is room for improvemnt when it comes to Persia and barbarians), although if we had the time and tools, overhaul mods would no doubt be on the list.
As for reselling and taking credit oneself, of course not. Just for your information, my background is Total War games - which have tons of mods, and the earlier titles are easy to mod. That should explain my interest in mods. All this being said, Hegemony Rome should keep me content for a long, long time.