At this time unfortunately not. The only thing you can do is just add the user to the group the subscription gives access to. We'll have an option to do this in version 8 however.
You could potentially add the user to the ubbt_SUBSCRIPTION_DATA table as well if you're comfortable with mysql. Looking at a couple of the other active subscriptions for an example it would be fairly easy to insert a new row so the subscription expires when you want it to.
