I dont think you can. You might be able to cludge a work around however. Make a blank menu ring with no items and capture the left click event. On this event display your own custom menu with sub items. Example of right click menu with sub items provided at link below...
http://sine.ni.com/apps/we/niepd_web_display.DISPLAY_EPD4?p_guid=DC94D017D9632DE4E034080020E74861&p_node=DZ52048&p_submitted=N&p_rank=&p_answer=&p_source=External