As far as my knowledge goes, you’ll have to write the extension for the Content Pipeline in order to load and process custom XML, or deserialize XML without using Content Pipeline. Content Pipeline’s default XML importer (again, as far as my knowledge goes) doesn’t support much aside from basic things like collections, MonoGame’s structures like Rectangle etc.
This is a good set of tutorials for Content Pipeline, despite being for XNA: http://rbwhitaker.wikidot.com/content-pipeline-tutorials