Thanks for the clarification. As the message in the module shows, you need to “Select master volume to enable editing”. See instructions here: Issue during voxelization of a model stl - #2 by lassoan
You can register surface of a segmented image with a surface scan using modules in SlicerIGT extension. You can either specify landmark points manually and compute registration using in Fiducial registration wizard module. You may also try to use Surface registration module to match surfaces automatically: for this you need a good initial position and surface model that does not have any extra/incorrect surface patches.