Do the expansion teams have at least 25 major league players on their roster? The game will crash if it doesn't have at least 25 players in each organization.
There must be an even number of teams in each league or else the game will crash (this is what Killebrew was referring too).
Finally check this <a href="http://www.ootpbb.com/board/cgi-bin/ultimatebb.cgi?ubb=get_topic&f=5&t=000008&p=" target="_blank">FAQ</a>. It details the process of adding expansion teams to custom leagues, I've been told it works almost perfectly (which of course made me smile
).
If you have any further questions on expansion feel free to ask, either myself or one of the older more wiser players will answer.
Hope this helps, good luck.