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