Last year it was announced that it would be possible to top off award certificates with up to 15,000 in points. For example if a property costs 65,000 points and you have a 50,000 free night certificate you can use the certificate and add 15,000 points. This feature is now live and the F.A.Q for this can be found here. Seems to be live on the website only, but should be available on the app soon. If you have a free night certificate that can be topped up, you should be given the option when booking.