Hello @puchonge, welcome to the forum. 
You happened to encounter a programmer who has read over and contributed to the source code of the MATE Panel. I can tell you that, the way the MATE Panel is implemented now, there is no way that I currently know of to add such a fourth menu to the Main Menu. (The FreeDesktop.org menu standards refer to other desktop environments; MATE does not have such a capability yet.)
However, if you really want this feature, I might be able to give you a "patch" to the source code which, when compiled along with the MATE Panel, adds this extra configurability to the MATE Panel. Eventually it may become an official feature of the MATE Panel, like many of my contributions thus far have become. However, it will take me hours and maybe even days to implement the feature, and you will have to recompile the MATE Panel with my patch (I outlined the full procedure here: How to modify and rebuild a package for Ubuntu MATE (or Ubuntu, or Debian)). I'm not sure whether you're willing to wait and then put in the effort to get this feature though, so until you say yes, I shall not start on this feature.
Let me know if you're willing or not to go through these hoops.