Certainly possible in theory. I don't know the product you refer to, but if can output valid XMl conforming to your DTD you can likely get there.
I have played with the XML import features, and there were limitations in XPress 5 with how sophisticated you could make the placeholder styling, the New in 6.0 doc says these are now more flexible. Have not played with this at all in XPress 6 (or export for that matter, I still use XPress 5 for what I do, which is convert print documents to XML, and not a ton of that though large files when I do them.)
As you probably realize, you have to be highly structured. The ideal case is a one-to-one mapping of XML elements to XPress style sheets; this is helpful in importing, and pretty much a must for exporting. And there are limitations in what XPress can do with entities and attributes in XML, so if your DTD is quite complex you may need to dumb down the XML you bring into XPress. (Again, based on pre-XPress 6 experience.)
The XML Import Guide.pdg covers creating placeholders and importing. The Guide to avenue.quark (in a different folder) goes over tagging rules and exporting. Took a quick glance at this and don't really see anything that looks different than in 5.0 for exporting.
All that said, not sure about the training part, but at this point you probably want to focus more on the feasibility of XPress for this project than the training side. If your XML knowledge is limited (as mine is, don't get scared) I'd suggest the XML Bible (see the author's site at http://www.ibiblio.org/xml/ for lots of good links and info) or Black Book, and if any of the XPress 6 books have a decent XML section they'd probably be handy as well. http://www.xml.com/ has links to a wide variety of free and commercial XML resources.
BTW, where you say:
>> export entire documents, complete with layout
this might be a little optimistic. XML does not generally describe layout, unless you plan conditional application of XPress templates or styles based on specific contained attributes. In that case it might be doable, but you'd need some middle piece like AppleScript to examine these files and choose that template. Also don't think any of this is really scriptable now, so unattended creation of layouts may not be possible.
Good luck, John