CE isn't a modding tool and I wouldn't recommend it being used that way. You can make very small changes and you really need to know what you're doing before hand. You can open a file and do a little hex editing. But once you change something, that change is instantly written.
The tool depends on the game. For example, Gibbed Tools for JC3, QuickBMS (widely used) for Witcher 3/MGSV... Do your research on the game.
As for your rom question, if you're talking about emulation, yes it can. Check Mem_Mapped under Settings>Scan Settings.