You can try a batch converting using Adobe Acrobat Professional (I'd recommend version 8 or higher), having the Adobe PDF printer as the default one.
Simply open a Explorer window, select the desired files, right-click on the selection and choose the "Convert to Adobe PDF" command from the contextual menu (the name of this command may vary, depending on the version of Acrobat).
BUT this procedure can be done only using XPress too, it will open QuakXPress AND each document, directly with the Print dialog box open. If you have broken links to certain picture files and/or missing fonts, you'll continue to receive error messages. I guess there is no way to pass this error messages without fixing the problems.
Keep in mind that this procedure is memory consuming, so try first with a small number of files (this mai depend on how big are the QXP files).
I know my answer does not help you very much, but it's the only one I know.
Every day is a good day to learn something new.