The Workplace Shell along with all its registered classes, be they system-supplied or user-written, is itself just a Presentation Manager program. As such it should be able to cooperate quite well with other PM programs, as long as it conforms to the protocols laid down by PM, particularly for drag and drop.
The technical term for the drag/drop protocols is Rendering Mechanisms; the standard ones are defined in the OS/2 2.0 Programming Guide Volume II. The Workplace Shell implements the Print and the OS/2 File rendering mechanisms. These make it possible for a PM application to provide direct manipulation printing capabilities and, if required, file drag/drop facilities similar to those provided in the OS/2 Version 1.3 File Manager.