How to add a new mage kit to the game ? This kit must be usable by the PC.

I think remove a MageSchool

You can add as many mage-kits as you want but to add one that the PC can choose at character creation you would have to remove one of the existing mageschools yes.

The only one I might consider removing would be Diviner.

One question: if you remove an existing mage kit to make yours, can you make it avilable to gnome (for example?)