Can I modify my installed Openlayers plugin files to add a new layers origin? If yes, what files?
Yes, it is possible by manually editing the python file
You have to set up a local server (like Apache by MS4W) if you want to use your own tiles as localhost.
Only disadvantage is that you have to do the adaptaion every time the plugin gets updated.
This is what I inserted:
And inside the myMapnik.html (a copy of osm.html):