It is not possible to replace water butter when making a cake mix. This is because the butter has more ingredients that can spoil the outcome of the cake. It would be better if you replace the water milk instead of butter.
You can substitute all of it! Instead of water use milk, instead of oil use butter and add 1 or 2 extra eggs instead of 3 eggs use 4 or 5. I make my cakes like this and love it, it's more moist I think lol. But use the same amount of milk and butter on whatever the box recommends. Good luck! I love making cakes!