Read KML files, edit items, and rewrite files?

[Winfried]

Hello,

From existing KML files, I need to read all Placemark items, and then add some elements to each:

• If they're waypoints, add a "styleUrl" element
  
• If they're LineString (tracks/routes, really), add a "Style" element
  

… and save the whole thing into a new KML file.

SimpleKML works fine to create brand new KML files, but how can I read existing KML files first?

Thank you.