Are you trying to serialize a custom class you've made, eg something that's not under MonoGame framework? If so, you need to build the assembly that contains that class as a dll and add reference from the Content Manager to it (and unfortunately, you can't reference your own exe, because the reference is used during compilation. you need to make an external dll).
Anyway, the following is an example of a working XML from one of my projects, just so you'll see a valid syntax:
<?xml version="1.0" encoding="utf-8"?>
GeonBit.UI.DataTypes.TextureData is defined in a dll I have a reference to.
Hope this somehow helps!
(I ignored the second part of your question because I don't know the answer of it, eg if the XML importer can be used to something other than serializing objects).