Officially there is no way to do it. But... what you can do is pierce the boot, inject grease into the area. Clean the boot with alcohol. Then put some Black or Red silicone sealant on the hole you made.
Only aftermarket joints have grease fittings on them. How nice of Ford to eliminate them huh???
It's not just Ford; almost everyone has eliminated the ability to add grease, so now you just have to replace the parts.
Also, I would not recommend using alcohol on the rubber boots, since it will dry out the rubber-just wipe the rubber with a damp (water) rag, and shoot it up.