I'm pretty sure you cant do this. I needed to do this as well and ended up using DTDonate. Wasn't able to add it to the registration though. The user had to go to a different tab to make donations. This would be a great addition!
I'm not aware of a way to allow someone registering to enter a specific amount they want....
You can easily add different donation amounts as conditional fields - ie have a checkbox called "would you like to donate?" and then have a heap of radio buttons appear if they check the first one -
$5, $10, $15, $20, $50 etc. each adding the appropriate amount onto the fee.
Not as flexible as what I think you are asking for, but it should still work.