Tricky... I hate asking people noobish questions so I'll just tell you how I edit those.
1. Open the .pkg with WinRAR
2. Find the wanted .xml file and double click it, thus opening it in IE
3. I open the sourcecode (into notepad) and edit the part I want
4. I save the notepad, at which point WinRAR notifies me that the file has been changed and asks if I want to add it to the archive. Click yes and after some processing I'm done

I think there's even a faster way to do this, and also you could just extract the file out into Dawnspire directory (with proper sub-directories) to avoid messing with .pkg (like the Master Server and Deathmatch patches are distributed).