User blog comment:Armenek/Kasumi on Mass Effect 3/@comment-730941-20100416084054

The difficulty will be that she's a character that not all (maybe not even most) players will have bought.

If I had to guess, I'd imagine that Kasumi will be referenced in ME3 with a cameo or perhaps just an email (that's if they don't allow you to keep old squad members from ME and ME2, which is a possibility), just like the 'Bring Down the Sky' DLC was referenced with a radio news report.