Yees, you create the .xinitrc file. I use the Terminal and a text editor that ships with OS X- pico.
From the prompt, make sure you are inn your home directory (~/). type "pico .xinitrc". Window will open, add the commands you need then ctrl-o, enter, and ctrl-x.
There are a number of properties you can add in .xinitrc for your paths and for X11.
There is a X11 mailing list that is very helpful.
HTH
Craig