AZSpudKing

I've been looking around for an answer to my question but can't seem to find one, sorry if this is duplicated. I would like to offer a promotion to my exiting clients for paying for more hosting time while they are still in there current hosting term. In other words, is it possible to have my customers prepay for their account, before they are due for renewal and use a promotion code that gives them a percentage off or even free months? For example, I have a client with a six month term and is now three months into it, can the client prepay for another six months now, have it tacked on to the existing term and receive a discount for doing so with a promo code? Thanks for any help or suggestions.