If you want to provide the X server with one or more environment variables and values when started by the login server, you can specify them using the Dtlogin*environment resource in Xconfig. For example:
Dtlogin*environment: VAR1=foo VAR2=bar
will make the variables VAR1 and VAR2 available to the local X server process. These variables will also be exported to the Xsession and Xfailsafe scripts.