There is currently no 64-electrode waveguard cap in the defaults distributed in Brainstorm. This is something you can look at yourself in the list of available EEG positions.
I don't know if the positions of the 64 caps are subsets of the 128 or 256 caps.
If they are, and if the electrodes names are consistent across caps, you might be able to use directly the positions of the 128 cap with your 64-electrode recordings (with the menu Add EEG positions).
Otherwise, the beginning of this discussion thread is applicable.