It is possible to add a coupon to an existing subscription to apply on its next renewal.
Go to Subscriptions
From the merchant portal, choose Subscriptions from the navigation menu.
Find the subscription you want to add the coupon to
You can use the search bar to search by customer name or email or the id of the subscription.
Click the Actions Menu in the upper-right and choose "Add Coupon"
Choose Your Coupon
From the dropdown find the coupon you would like to apply.
You should see something like this:
IMPORTANT: Keep in mind that this coupon will not be applied immediately and will only be applied upon the next renewal.