Yes, you can add a control group after a campaign has launched. To do so:
- Open the campaign details from the 'All Campaigns' page
- Click the button on the right side of the page below the campaign copy
- Click the button and move the slider to allocate a percentage of the group to be in the control.
- When you are finished making changes, click the button to return to the campaign details page
- Finally, click the button at the bottom of the page