I dont know if you tried it yet, but kmenuedit seems to be the easy way:
Stuff will be saved in ~/.local/share/desktop-directories/ so its a way to do it in $HOME 
Furthermore I think customisations are overwritten whenever this package is upgraded:
╭─nico@T440p in ~ took 1ms
╰─λ pacman -Qo /etc/xdg/menus/applications.menu
/etc/xdg/menus/applications.menu is owned by kservice-git 5.80.0_r874.gfc86866-1