There is a workaround to change manually the z value of your volume vertices:
Draw the volume in Volumes module and edit the height of the vertices in rayCloud. When the volume is drawn vertices are created as MTPs. These MTPs can be modified to 3D GCPs (Ground Control Points) or Control point in the rayCloud view. This enables you to give specific coordinates to each point (z), e.g. based on a previous survey of the area or as a height of points in the surrounding area from the rayCloud. You should then save, close and reopen the project, go to Volumes module, use Triangulated (default) processing option and calculate the volume.
I hope this helps,