I've been in this same situation. I had a $100 voucher given for damaged luggage, and I was ready to use it, so I called up and they told me that if I wanted the double points and to avoid the extra phone fee (it was $5 cheaper online at the time - not sure if this is still the case), I could book it online, and then call back, and they'd refund, in the original form of payment, the voucher amount. I was a bit skeptical, as I was laying out the money, and then relying on the next phone agent to apply the voucher and refund it, but it was actually very easy - the rep knew exactly what I was trying to do, and there were no problems with it at all.