Ted,
Buy-in the players who haven't shown up yet, but don't mark them "Paid in Full". Then you can seat them and start the tournament, and post-and-fold them until they arrive. Mark them "Paid in Full" when they show up and pay for the buy-in.
Allowing rebuys before a player has busted out is on the to-do list. In the meantime, you can use the add-on feature to get around this, as you can purchase an add-on while a player is in the tournament, and you can adjust the values (cost, chips received, etc) for the add-on at time of purchase. Now, this *will* skew your stats if you are tracking add-ons vs. rebuys, etc., so, FYI...