I didn't think that was what that section was for, but it tried it, and it worked. You woudn't be able to assign an add-on when you actually buy-in the player, but you can assign them an add-on from the tournament screen before you start the clock if you've got it set that way.
An alternate way of trying this, is to do your buy-ins in excel or some other spreadsheet program, then import them all at once. In the old version, you could put something in the add-ons column this way.