Usually if people put them in, they add them to the ball joints first. They aren't that hard to install. You just drill a small hole in the bottom of the stock ones, and screw in a grease fitting.
Factory units are sealed. Factory ball joints and tie rods on 94+ stangs last a long time unless the boot gets ripped. If you want to grease them you can buy a needle off the snap on truck that hooks up to a grease gun.