Currently, supported bitmap font format is .fnt.
Just set Importer name to BitmapFontImporter and Processor name to SpriteFontProcessor, like below.
<Item Include="\Fonts\petangue-42.fnt">
  <Name>petangue-42</Name>
  <BuildAction>Compile</BuildAction>
  <Importer Name="BitmapFontImporter" />
  <Processor Name="SpriteFontProcessor" />
</Item>
You can create custom Importer library .dll to support .xml bitmap font format.