Is it possible to make python addon as a modifier?
I just would like to make some deformation modifiers. But I know only python.
No, the modifier API is currently not exposed via Python.
Arguably, Python is too slow to do realtime operations on mesh data which can have millions of faces.
Though there is some possibility of something faster using Python, NumPy for example or maybe eventually even PyPy.
As of 2019, it is still not possible to write new Modifiers in Python, and this is even listed in the "Anti-Features" on the dev wiki, so don't count on it.
Nevertheless, such modifiers can be written in C or C++ and for people who have never touched the Blender code base, I wrote a guide focusing exactly on this: Writing a custom modifier for Blender