Yes! Well, it's a bit of a workaround, but you can can do it pretty smoothly.
- Go into the customer's profile page under People and notice the next charge date for their subscription plan and how many days exist between now and then
- Delete their current subscription
- Re-add them to that subscription
- Edit the subscription to push the next charge date out x days (the number you got from step 1)
- Go back to the Access tab and click on Apply Coupon to apply the code