As Father Bronze said... in a full game, in the hut in Kuldahar that was previously locked (if you didn't have HoW), sits Hjoldar. You talk to him and assuming your party is level 9+, you can go off to help in Lonelywood. Get dropped off there. Clobber your way through it. And after the final fight, he brings you back to Kuldahar to finish your quest there. That's the full game option.
So, once you fight Polquin in Lower Dorn's, it is too late to go to HoW, but anytime after you've left the starting town to before you've returned to it, assuming you meet the level requirements, you can take off to deal with the HoW.
As for the exported characters, IWD1-6 are the auto-exported party members from the final IWD fight. HOW1-6 are the auto-exported party members from the HoW fight. Nice advantage of playing the full game is your characters actually get to keep the xp from defeating HoW.
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