If you look at a .user file it's just an xml file, perfectly readable in any text application. .stock files, however, are zip archives, not just an xml file. However according to
this Wiki article presumably a .stock file could also just be a portfolio, so maybe a multi-character portfolio would work, or maybe you would make multiple portfolios and zip them up. I think the only way to really find out is by playing around with that a bit.
If you figure it out, however, definitely let us know what seems to work!