check the current layout
$ setxkbmap -query
rules: evdev
model: pc105
layout: de
variant: qwerty
$ sudo cp /usr/share/X11/xkb/symbols/de /usr/share/X11/xkb/symbols/de-ori
$ sudo vi /usr/share/X11/xkb/symbols/de
search for the right section, in my case xkb_symbols “qwerty” and add the following line:
key <AD12> { [ plus, asterisk, dead_tilde, dead_macron ] };
$ setxkbmap -layout de -variant qwerty
AltGr+~+n => ñ
reference: https://help.ubuntu.com/community/Custom%20keyboard%20layout%20definitions