I've done this recently in Build 6310 of COMPRESS for a vertical vessel with a conical bottom.  I just created the nozzle like a typcial nozzle. I chose the option to locate the nozzle "From the Datum Line" and entered an elevation that placed the nozzle somewhere (anywhere) on the transition.  This will change the top right part of the Nozzle Data window to let you indicate the nozzle is parallel to the vessel centerline.  By indicating the nozzle is parallel to centeline, the 'Distance (r)' field then becomes the offset distance from the vessel centerline.  The value you enter here for the offset distance will force the elevation you previously entered for the 'distance from datum line' to change to an elevation that is coincident with the 'offset from centerline' distance at the transition surface.