Sorry, don't use Shoebox - couldn't help you there.
If you don't figure that out, why don't you simply write a bit of code to either:
- convert the font xml to a bmfont xml. Looks straightforward enough.
- write an importer to this file format type in tk2dFontBuilder.cs. If you look at the XmlImporter code, it should be easy enough to create a variant that will read this format as opposed to bmfont. They contain mostly the same data, so it should work fine.