This is not currently a feature within Blackbaud Sphere. 

A work around for this is as follows:
1. When entering the sponsorship change the sponsor amount to $0
2. Then enter a donation through the bookkeeping>gift entry from the sponsor in support of  the participant with the sponsorship amount
3. This will allow for a donation from the sponsor and they will still be displayed as sponsors for the scroll box and view our sponsors page
4. When running sponsor reports in the event be sure to include the amount column so the $0 amount is counted and not just look at the level and think that the amount should be included 

If you would like to suggest that this be a feature enhancement for a future Blackbaud Sphere release, Blackbaud has an Idea Bank that clients can go to and list any suggestions they have for our programs. Go to http://sphere.ideascale.com/ to enter suggestions, and vote for favorite suggestions as well. This is a direct line to our programming team and they look at this for enhancements to the software.