This reference documents NEDXML -- a library for parsing NED and MSG files, and much more. The result of parsing is a DOM-like object tree, which you can export as an XML file (and import it back as well), generate C++ code from it (a la nedc), or -- possibly after modifying it -- convert it back to the original NED/MSG format.
For a start, read the Overview, or you can go to one of the more specific topics: