How to convert a Blogger XML file to a WXR file?
Do you really need to do this at all - according to the Wordpress Codex, you can import from blogger directly
The pertinent bit from the link above says:
Users of WordPress 2.2 and above can import from Blogger. Earlier versions cannot import from Blogger because "New Blogger" was released by Google, invalidating all the old importers. Only WordPress 2.2 and later has support for "New Blogger".
If you haven't already, you must be using New Blogger and a Google Account on Blogger. If you are still using Old Blogger, the importer will not work, but there is a helpful online tool that converts legacy Blogger Atom exports into Wordpress WXR export files.
To start the Blogger importer, go to Tools → Import → Blogger. For WordPress versions 2.0.2 to 2.6.5, go to Manage → Import → Blogger.