Just to clarify: you are referring to Notepad++'s User Defined Language as to “xml file”, and your question is whether one can edit the contents of Mathematica notebook visible on screen with tools for editing notebooks themselves (tools like Notepad++ with user defined language).
Yes, it is possible. Actually, it is an important feature of Mathematica's language: what you see in notebook is the same material notebooks themselves are built with.
Two pictures are worth two thousands words:
This is notebook itself (what you don't see on screen) edited in Notepad++:
(You may view the image above in a separate tab to enlarge it in case it's shrinked too much.)
Below is an expression from the notebook (what you see on screen) edited in Notepad++:
NB: Notice, however a small drawback: “n” is printed black in Notepad++, as if it was a standard function. It's actually
N that is standard fucntion but Notepad++ does not distinguish upper and lower case (or it does but I failed to find how it's done).