Yes, you can add custom email opt-in text for a TeamRaiser per participation type. To change this:
  1. Go to Fundraising > TeamRaiser
  2. Next to the TeamRaiser in question click "Edit"
  3. Go to step 6, "Manage Participation Types"
  4. Next to a participation type, click "Edit"
  5. Go to step c, "Select Registration Info"
  6. At the very bottom you can enter custom text for the email opt-in checkbox where it says "Custom Opt-In Text"