Yes, it can be lubed. But if a ball joint's "boot" is torn, the ball joint must be replaced. The whole reason for the boot (and the grease in it) is to protect the metal components of the ball joint from corrosive elements. The noise you are hearing is a result of the torn boot and it's resultant exposure to the elements. A new ball joint should take care of the noise, assuming nothing else is in need of attention. Check the other side also. If that one isn't torn, it's a good opportunity to lube it. This will help keep it healthy.