Locate the control_name of the control in the Main Panel to which you want to attach the subpanel.
If the control is one of the built-in controls, its definition is in /usr/dt/appconfig/types/language/dtwm.fp.
Create a new file /etc/dt/appconfig/types/language/*.fp.
Define the subpanel:
SUBPANEL subpanel_name { CONTAINER_NAME control_name TITLE value KEYWORD value … }
Save the new configuration file.
Choose Restart Workspace Manager from the Workspace menu.