If your unstructured FrameMaker is properly styled with paragraph tags, you can create a conversion table to convert it to structured FrameMaker. Then save it as XML. The conversion process is usually not clean, so you will need to do some cleanup in structured FrameMaker before saving as XML. Creating a conversion table is documented in the Structured Applications Guide that comes with FrameMaker, or you can download it from Adobe.com.
Depending upon the amount of data you need to extract, this may or may not be the best method. You might look into creating a PDF of the FrameMaker file and then see whether the extraction features in Acrobat might get what you need. You can save a PDF as XML, but I do not know how good it is.
Another option would be to use Leximation’s MIFML converter. It transforms the MIF files you can save from FrameMaker documents to a valid XML format. From there on you can build an XSL transformation to get the XML structure you want. I have done this, but it requires a certain amount of documents to be worth the effort.
PS: To clean up the result of a conversion with FrameMaker’s conversion table feature I find West Street Consulting’s plug-in FrameSLT indispensable.