There are lots of XML files used in SL but I wasn't aware that they stored any geometry- and I assume this is what you want to convert?
If we could write 3D models into xml files and load them into SL, it would be great but, as far as I know, this is not possible.
Perhaps try AC3D's Second Life Tri-creator - this will let you get any 3D model into SL but there are many things to beware of- see the notes.
http://www.inivis.com/secondlifetri.html