Dear Robert,

as you say, SDM only deals with voxels. However, you can use external tools such as mri_surf2vol / mri_vol2surf from FreeSurfer to resample surface into volume and viceversa. Remember to specify or transform to the correct stereotactic space (e.g. MNI), and that the SDM mask should only include the gray matter analyzed in the surface-based morphometry software!

With the best wishes,

Joaquim