"How can I convert an XHTML form into an XForm that uses an XML file as a data store and is usable in a web browser" Thanks to David Dorward for correcting my question formulation.
XHTML IS XML
well, not exactly. But there is no real difference. XMl is just a set method for describing data in a fairly lose and abstract way that is also nicely controlled. The idea being that XML data is very easy for any XML aware application to read in a work out what is going on.
XHTML is a (roughly speaking) subset of XMl - it is a specific use of XML. It is HTMl written in an XML way.
I know I have said XML and XHTMl a lot here... but its dam hard to explain how these are the same and different at the same time
Valid XHTML may not be XML due to the mime type it is served with, but valid XHTML is a subset of the XML syntax. That said the perfect solution is to ensure your XHTML is treated as XML by serving it with the mime type application/xhtml+xml instead of text/html.
If that is not a problem you have to supply a XSLT stylesheet to transform your XHTML form into XForms compatible elements, or simply serve XForms type form in your XHTML document directly. I recommend the later of those two solutions.