There are some native javascript parsers.
http://goessner.net/download/prj/jsonxml/
and there is also an own jquery-plugin.
But keep in mind xml code can contain characters or structurs which would be invalid in json so the converting can fail!
So the best solution is to extend the API to return also JSON-Data if you want.
Nowadays nobody would build a xml-API for Web-/ Mobile-Apps.