As far as I am aware XFDF data is collected in one of two ways, either form data is submitted to a server (submit button on the PDF form), or users themselves export the data using 'Advanced'->'Forms'->'Export Forms Data' option in Adobe Acrobat, so regards exporting data, I guess it depends on how the PDFs in question are setup.

There are posts here about populating PDF forms (super search) which you should be able to use to get you started filling in forms with existing XFDF data.

Martin