Modding

maybe... just hypothetically, you could extract the graphical part from the original, hype it up, and put it in Unreal... but how about the part that is, hmm, the strategy itself, all these moves, turn-based battles, skills, etc.? Is it realistically feasible, and if so, how?

Actually, "hyping it up" is unlikely to work. The whole point is that the models in Heroes 5 are very low-poly. Their beauty and quality lie in the artistic talent of those who painted them, and they have their own unique "style."
You can hype it up, but the model will end up looking like the one in Heroes 7, not like the one in Heroes 5.