Is it a good choice to add an extra move to primeape?

I would say no. 3 of its charged moves are Fighting-type, with one obviously superior to the other. Night Slash is poor too.

If it is your favourite mon, go ahead. If you can have both night slash and a fighting move, you can deal with psychics with the dark attack, while still able to deal with those weak to fighting normally.

My favourite mon is my snowy Castform named Cryo and I intend to give her a second move, I know it is not greatly useful and even needless since only ice moves are available, but I love the mon and it is fun to be able to fire up ice beam or blizzard.

Ofcourse, for straight-up powergaming, there are better mons to spend dust on other than Primeape and Castform. In competitive play they are both only dex-fillers.