International characters (useful e.g. for Spanish áéíóúñ)
are available via the flag -variant altgr-intl for setxkbmap,
see resulting keymap file with e.g. setxkbmap -layout us -variant altgr-intl -print.
Looking at the result of this command,
we see we just need to add (altgr-intl) to the us layout, making the symbols line now:
xkb_symbols { include "pc+us(altgr-intl)+inet(evdev)+customLayer(Fn)+customGreek" };