Userscript Submenu for GM Menu Commands
Include this module with
// @require in a script and then
call with parameters,
Then make calls to GM_registerMenuCommand as usual after having registered the module in this way. They will now be redirected to a submenu available by clicking on the Tools-->Greasemonkey or by pressing alt-m.
See header of script source for API more details.
For an example usage see script simple form saver at openuserjs.