I am not up on how much weight you can put up there.....I would not want to put much as weight up high is not a good thing for handling capabilities for any vehicle.
Also....anything you put up there adds to wind resistance, which also will impact the handling.
However, a luggage rack was an option on the windstar, so the roof must be up to something up there.
Having one added is possible....but make sure that it is installed correctly......you don't want any leaks into the vehicle.