Except that doesn't work with the way D:OS is designed. What if you import a level 1 into a game where the host is almost finished? There's no possible way for your character to level up. Or the inverse of that, what if you import a level 20+ into a new game? You're going to steamroll through everything. D:OS is not designed to support importing your characters, that's why you can't.
The OP is talking about persistent worlds aka MUD/MMO style, not campaigns. The Neverwinter Nights series only shipped with campaigns, but dedicated modders built their own worlds from scratch. D:OS is identical in this regard; it only shipped with a campaign, but people are perfectly capable of building a world from scratch.
The issue is not that the campaign is not a persistent world, but that there are no dedicated server executables and other engine features that are close, but not quite enough, for persistent world capability.
Whether it is feasible that the engine could be expanded to open this new world of possibilities is unknown (to me, anyway).
