Hi Asyrin,
There are two easy ways to add skillbooks to existing merchants.
First, you could add the new skillbook to an existing treasure table. Look up which treasure table the fire spell merchants use, copy that to your mod and add your skillbook to it. If you don't change the name of the treasure table it will override the existing one.
Second (only if your project has a dependency on Origins), you could open the level that contains the merchants and add a second treasure table.
Hope that helps!
Last edited by Larian_JB; 21/09/17 07:16 PM.