The buy-in, add-on, and rebuy amounts you set on the Game tab of the Settings page are "default" amounts. For any buy-in, add-on, or rebuy, you can override the amount.
So, yes, the software does support it in the sense that you can change the cost of the rebuy for any player at the time that they rebuy. The Rebuy dialog lists the amount of rebuys each player has purchased, so you can see which [numerically] rebuy any player is purchasing.
It doesn't support it in the sense that it will automatically change the amount for you based on how many rebuys a player has already purchased. It's a manual thing.