Hello,
You cannot use importing for editing but what you can do is first export the coupons in awocoupon->tools->reports->coupon list. Then make the changes to the expiration column. Then delete the current coupons and then upload.
The down side to this if if some of the coupons have been used and you have set restrictions or uses or they are gift certificates, then those will be lost as a new id will be created for the inputed coupons.