I’m don’t know if Notepad++ will be a great deal of help here, though I’m not sure I follow your proposed workflow.
I take it you have a bunch of media with Kodi-style nfo files beside the media files. What program produced the nfo files? (If it happens to be Tiny Media Manager, I know that program can export a CSV or similar file from selected fields. That file could then be manipulated in Notepad++.)
There is an XML Tools plugin for Notepad++. I don’t use it, so I’m not sure if it has the capacity to work with an entire directory tree of xml files.
Unless a facility like the one I mentioned for Tiny Media Manager is available, most likely you would be better using a scripting language, like Python, to discover the files, read them as XML, and write a file containing the information you need to process. You could then load that into Notepad++ for editing.
(I’m not saying it’s impossible with just Notepad++, but it will be slow, annoying and error-prone.)
I’m trying to add tags to my .mkv and .mp4 files
That will require some sort of scripting. If it’s something like a batch file you’re planning to write, with a line for each file calling mp4box for mp4 files and something else for mkv files, Notepad++ could be of help turning a file with one line for each nfo file into a batch file to update the corresponding media files.