You can give the option for your Participants to make an additional gift/donation at the time of registration by enabling the 'Would you like to make an additional gift?' section during the registration process.  
  1. Go to  Fundraising > TeamRaiser.
  2. Find the TeamRaiser you want to change and click the Edit link.  
  3. Go to 2.Select Fundraising Options and then look for the option labeled ' 4.  Additional Donation with Registration'.  
  4. Check the box ' Yes, display the Additional Gift field on the participant registration forms'.  You should now see the    'Would you like to make an additional gift?' section during the registration process.