Just create and export the six characters you want. Then restart in Multi-Player, tell it serial connection (IIRC,) but give it nothing to which it can connect. When you get to the screen with the six empty character boxes just click each one and import the character (or characters, if you just want to duplicate one) you want. If you do not wish to go through Multi-Players extra load screen each time you can just save the game once you get your charcters in then quit, go into the BG file and change rename the "MPSave" folder as the "Save" folder, then go back to playing in Single Player.

You can also duplicate unique/rare/expensive/limited items (e.g. stat increasing manuals/tomes) that way, though obviously it is somewhat unethical (and gamebreaking to have six people running around with Drizzts scimitars. ) I always do that with the boots of speed because it is a royal pain to zip my ranger to the edge of the screen then have to wait for the rest of the party anyway (Baldurs Gate: Where "never split the party" is not just a rule, but part of the code. ) But remember: Experience is divided by the total number of classes in the party, so if you load up with a half dozen multi-classed characters it will take you a while to get anywhere with them. And you still will not get far because the XP cap is so low on BG1 (though TotSC will bump it one level; personally, I found Durlags Tower the most fun part of the whole series, but that stupid island was the most annoying.)

Anyway, hope that helps, assuming you are not through it already; have fun.