Use dtaction in the EXEC_STRING of the action.
For example, the following action uses a built-in action named Spell (the action is labeled ``Check Spelling'' in Application Manager). The new action runs Text Editor and the Spell action, displaying the spelling errors in a separate terminal emulator window.
ACTION EditAndSpell { WINDOW_TYPE NO_STDIO EXEC_STRING /bin/sh -c 'dtaction Spell \ %Arg_1"File:"%; dtpad %Arg_1%' }