Is there possiblity to write my own plugin/extension to Unity editor ? I want to write plugin to generate map for me from xml file.
"Plugins" in Unity nomenclature mean native code (read: written in C/C++/Objective-C) DLLs. For what you want to do you more than likely don't need (or want, really) to use plugins.
For extensions, the Unity editor is very scriptable.
Are you asking if you can
Either way, what you're asking is more than possible. Here's an example of somebody completely replacing Unity's binary scene file format with a text representation: https://github.com/terravision/UnityTextScene
What you probably want to do is more likely just take advantage of their editor scripting tools. For example, look at ScriptableWizard. You can take that base code and then do something like load an XML file and spawn a bunch of prefabs, or whatever.
Pretty much the vast majority of things you see in the editor is scriptable. You can attach components, move/rotate/scale things, spawn prefabs, edit materials, all sorts of things.
Yes - look on the Unity Manual:
The manual also points you to further reading by providing things like the simplest plugin project - you could use this as a starting framework
Chances are that you are using the free version of Unity. As such, you cannot write plugins unless you use the Pro version.
If you are using the Pro version, then check the manual.