This is not currently standard functionality in Sphere. 

If you working within a Kintera Thon, you could enable a static additional fee available for selection.
To enable this option:
  1. Navigate to Communities>Special Events>Friends Asking Friends>Kintera Thon
  2. Hover over the event and select Website Info
  3. Click Fee Structure
  4. Mark the corresponding Enable Additional Fee box
  5. Enter a description for the Fee, for example "I'd like to cover Credit Card costs.."
  6. Click Add Additional Fees, which acts like a save button
  7. Navigate back to the Webinfo Checklist and click Publish This Event Now