the lower ball joints are noisy and cause slight clicking when driving. Here is my questions, is it possible to grease them or should they be replaced?

 

Are you getting excessive movement in the ball joint?

 

If there's any movement, then replace them. Are you sure the sound is coming from the ball joints (not from the CV joints or wheelbearings)?

It's possible to re-grease them, and you can buy new rubber boots for them too. But usually if they're making noise it's too late for that.