Additional properties for the wp_user class

Hi,

what would be the best way to extend the wp_user object with some custom properties? e.g.
$currentUser = wp_get_current_user();
$firm = $currentUser->firm;
$Position = $currentUser->position;

I basically just need a few additional custom properties (retrieved via a webservice) that I would populate on the fly upon user login and can interrogate site wide as needed. If this is considered very difficult/unwise, can anyone suggest an alternative?

thanks

Matt