[Freesurfer] creating a gca file from .mgz files

From the gcaFormat freesurfer wiki page I know that: The .gca file is composed of three volumes, which can be extracted like this:mri_convert atlas.gca -nth 0 means.mgz

mri_convert atlas.gca -nth 1 labels.mgz mri_convert atlas.gca -nth 2 priors.mgzwhere:means --> the mean intensity of the most likely labellabels --> indices of the most likely label at each voxel (indices refer to '#No.' column of $FREESURFER_HOME/FreeSurferColorLUT.txt)priors --> the probability that that label occurs at that voxel Is it possible to go the other way? I want to create the labels.mgz file and the others manually, and turn that into the .gca file with mri_convert. Can this be done? Thank you, Jeff Thompson