Hi again,
I see more of what you are saying now. My guess is that this is a bit like the data modelling functionality. When you stereotype a class as a <<table>> EA treats it differently, and brings up a different dialog than with 'ordinary' classes. Thus, you can create columns without having to stereotype them by hand, and enter meaningful values into them.
This capability is beyond the basic UML functionality (although perfectly 'legal' of course). EA handles the situation by invoking an alternate dialog directly. Unfortunately, at this time (or last time the Sparx folks answered a question about this, not too long ago) the function resides entirely inside EA, and is not accessible to the user.
If the way they've approached the XML stereotypes is anything similar then you are probably out of luck.
If you really need this, and are willing to do a bit of work, you can achieve much or all of what you want with an add-in, captuing events and handling the dialogs yourself. It does mean some front-end work, but you would have very fine-grained control, and you could also extend the basic EA functionality.
David