An eCoupon isn't a discount/promo code for a % off, it's like a gift card, reducing the cash you pay dollar for dollar.

OP, if you have an eCoupon that's about to expire, you can preserve it indefinitely by simply booking a new ticket using the eCoupon, then cancelling it in exchange for a gift card that never expires. You'll have to put up a few bucks out of pocket, but it shouldn't be hard to find a fare that's just a couple bucks more than the value of your eCoupon.

If what you actually have is a promo code that takes a percentage off the base fare, then what The Lev said applies.