You can use the 'Additional Fees' option:
  1. Navigate to Communities > Special Events > Friends Asking Friends > Kintera Thon 
  2. Hover over the event
  3. Select Website info
  4. Click Fee Structure
  5. Mark the box to Enable and fill out Additional Fees option.
  6. Navigate back to the Webinfo Checklist and click Publish This event now.

Note: This option will only allow you to purchase up to two additional t-shirts. Each additional fees (two of them) will have a fix cost. Registrants will have no control on how many extra t-shirts they want.