There is not a donation feature in DT Register. The only way is to enable partial payments for the event. Set that as optional with a minimum payment amount. This would work for you but you'd need a fee to be calculated I believe for that to work correctly. You could try it with a $0 event and minimum donation of $0 and see if that works. That's your best bet.
I have created a separate registration all together just for sponsors... set it at 0.00 as previously suggested and then create all your fee based fields. I have it set to invoice only, but if you are wanting to accept credit card payments, you could create a set of checkboxes all with different values. If they want to donate a custom dollar amount of 125.00... they can check 100.00 and 25.00 ... I know it sounds silly but if you type in every amount in increments up to your highest value you choose to accept.. the form could get a little messy. It is the only way I can think of creating custom amounts as I have not figured out a way for someone to type in an amount to be charged right away.