I haven't played Morrowind, but I gather that the NPC's there didn't have the multiple AI packages that an Oblivion NPC has. A merchant in Oblivion has a specific package where he'll barter (usually when he's in his shop) and he won't at other times. That makes it much more complex to make the change, because the non-merchant NPC's would need similar adjustment - "not now, I'm trying to sleep". Merchants also have inventory that's not on their person, although they can still sell from their pockets.
I'm not suggesting it's impossible, but it's probably a lot harder to achieve than the Morrowind mod.