While it's possible to add a "Donation" registration fee, it's not possible to change the amount.  Since additional donations are so poorly implemented in RE/BBNC, I add "Donation" registration fee to every event I set up.  Then, when an additional donation does come in, I add the donation amount to the participant record, as a registration fee, and then distribute the total.  This way, the event manager only has to look in one place for all payment.