Use DtWsmOccupyAllWorkspaces().
In occupy.c, the callback allWsCB() for the Occupy All Workspaces push button calls this function.
DtWsmOccupyAllWorkspaces (XtDisplay(toplevel), XtWindow(toplevel));
where:
XtDisplay(toplevel) is the X display.
XtWindow(toplevel) is the window to be placed in all workspaces.
See the DtWsmOccupyAllWorkspaces() man page for more information on this function.