When registering from the backend, you can register a person as a Participant and then as a Sponsor (Donor). Just follow these steps:
Click Register Person from the bottom left, and then select Participant when prompted to choose registration type.
After you've finished registering the person as a Participant, click the blue link that says, “Yes, register [Participant’s Name] as a Sponsor (Donor)“ in the confirmation window (as depicted below). When you choose that, it will autofill their information on the next Register Sponsor (Donor) form.
Then, once on the Register Sponsor (Donor) form, fill out the remaining Sponsor (Donor) registration fields (as shown below).
4. Click Register to complete the Sponsor (Donor) registration and add the donation.
