How can I add/modify layer to editor via code?

You can't, sadly.

While you can change an object's assigned layer through code, you can't define additional layers and/or tags, much like you can't change the Input$$anonymous$$anager's settings (a much needed feature, I$$anonymous$$O...)

Sad news.. thank you.

Old question but just in case someone reads it and think it is not possible. You can do it via editor scripting. I've written an article on this, http://www.plyoung.com/blog/define-unity-layers-in-script.html

[edit] Since my blog is no longer available I did a code sample on the Unity forum -> http://forum.unity3d.com/threads/adding-layer-by-script.41970/#post-2514259

• Nice. Everything's possible with reflection :)