Add a custom Cinnamon Menu Folder for a user:

mkdir ~/.local/share/desktop-directories

vi ~/.local/share/desktop-directories/customfolder.directory

[Desktop Entry]
Name=CustomFolder
Comment=CustomFolder applications
Icon=custom-icon
Type=Directory

place icon png file into:
~/.local/share/icons

sudo vi /etc/xdg/menus/cinnamon-applications.menu

add

  <Menu>
    <Name>CustomFolder</Name>
    <Directory>customfolder.directory</Directory>
    <Include>
      <And>
        <Category>CustomFolder</Category>
      </And>
    </Include>
  </Menu>

Adjust application desktop file to use the CustomFolder:

vi ~/.local/share/applications/x.desktop

add

Categories=CustomFolder;