1) You could start a multi-player game and give yourself control of all party members, just as in BG2 when you want to create your own party. This is probably closer to the "pure" IWD experience. It's best to start a game, save immediately, quit, and move the save-game from the "MPSAVE" folder to the "SAVE" folder.
2) The IWD-in-BG2 fixpack/tweakpack ships with twelve joinable NPCs. These are BG1-level (so no banter, not many interjections) and are built out of the portraits and sounds that come with IWD (as well as using bits of class-specific IWD dialog). You can get these NPCs by installing the appropriate component of "ice-tutu-tweaks", the tweakpack that ships with IWD-in-BG2. Seven are in Kuldahar, five in Easthaven.
3) You can use a third-party mod that adds joinable BG2-style NPCs. Currently the only available choice is Kulyok's IWD NPCs mod over at Pocket Plane Group (you need the IWD-in-BG2 version).
Edited by DavidW, 29 December 2011 - 05:51 PM.