In workbench, you can insert command snippets to use standard ANSYS commands. Named selections are parsed into the input file as components. This allows you to use the cmsel command to select groups of elements (if its a solid body) or a group of nodes (if it is a face). In the case of CERIG if you wanted to be able to select a point mass as your master node, you would use something like:
mnode = node (X,Y,Z)
where x,y,z is easily obtainable from your point mass description in workbench. I hope that answers your question.
Since I don't know what your working on, you might want to investigate remote displacements and being able to set materials behavior to rigid.