I got it. From the "Options -> Work Environment" menu, select the "Menus" command. There is a "manual" menu builder.
Every add-on exports the menu items implemented in the .grc file. Each of these items are recognized as "commands" by the Archicad compiler. By default, Archicad builds the menu exactly as described in the .grc file, with the exact same scheme. What I just found in the menu builder is that all of those commands can be placed at will, therefore is possible to rebuild the menu however I want.
Anyway, this is a very crude and dirty method to do the job, mainly because my add-on should export 14 menus with 20 to 50 commands each. So .. you get the point. I wonder if there is a more "delicate" method to create a menu at run-time, with the properties I presented in my original topic above...