Say
M-x customize-group
and
tex-command
Then find the "Tex Command List" option. From there you can introduce new AUCTeX commands.
However, AUCTeX is meant to be used a little differently. Instead of creating a separate XeLaTeX
command, you can do
M-x TeX-engine-set
followed byxetex
or(TeX-engine-set 'xetex)
in lisp code orCommand | TeXing Options | Use XeTeX engine
in the menu bar
(the three methods are equivalent).
After that the LaTeX
command will run xelatex instead of latex.