Why would you want to? Mask of the Betrayer is much better than the original campaign anyway, play through it once and you've little reason to go back. Or at least so I've thought, I know there are a couple people quite fond of the OC.
Anyway, yeah, expansion content in the original campaign. You're in luck, there's a mod for that... erm, or at least there used to be. The only one I can find now is for the second expansion, Storm of Zehir. Hmm.
Link. Basically converts the original campaign into the second expansions ruleset and added improvements, plus a few fan-made adjustments and tweaks for a more smooth and enjoyable experience. I
thought there was a similar mod with Mask of the Betrayer, but I can't seem to find it unfortunately.
Most of the thing MotB does are focused towards ridiculously high level characters anyway, aside from some races and prestige classes added in, there were minimal other changes especially at a lower level. Should be able to use that anyway though, just load the game up via MotB launcher and select the original campaign, that stuff should be there, though I don't think I've ever actually checked myself.