Umm, I'm not sure if this will help you.
In the "Virtual Path Regular Expression" field put ^/.*
Set "If this rule matches" to "Perform an external redirection"
In the "Redirect to" field put the url to the new domain
Uncheck "Append Query String" (Not sure if this is needed or not)
And set "Next Action" as "Stop matching"
in condensed form
^/.* -> Perform an external redirection -> http://newdomain.com -> Stop
Hope that helps!