It is based on the Custom Class, try to find one which let you choose at which % of mana it switch to wand or if there is none follow the guide to create a CC and modify the code of the one you use.
Can you advise where i can find one that allows me to choose this option? I have been looking for a while without success. if you can advise on how to istall it too, this will be appreciated
You mean which custom class or where to configure it ? Also, if you are playing a warlock, you don't need to use wand, just wait for lvl 50-60 and you will always have max mana.