Can I use a joker to help my partner after all my pegs are home safe?
I would say you can. It doesn't say you can't in the rules but it does say once you have finished you can help the team member to your left that is still going.
"Each player must play his own pegs until all five pegs are in the SAFE position. Then he can help his teammates. When playing with six players, you will help the teammate player to your left, if he still has playable pegs."