Actually there is a way to get a Quark file in to Word but it's clunky at best and produces very strange results.
Make a PDF of the file, open that in Acrobat (I think you need Pro) and export the file as a Word file.
Like I said the results are a mess but it'll achieve some level of translation.
That's one I would have mentioned, but based on my reading of the original post it sounded to me as if the poster wanted the editor to be able to make changes to the XPress document inside of Word and still essentially keep the document as an XPress document. That isn't possible.
You can indeed get a messy Word doc this way, but it won't be an XPress document and all you could get back OUT of it would be the copy and you would have to reimport the changes - so my though was, what's the point of that. Just save out a Word doc of the text boxes in question, have the editor make the edits and then reimport those.