What I do sometimes is create a new db entry, with the key prefixed by the user object key. The core never knows it exists, so it should always be forward compatible.
db.set('user:' + uid + ':something', "somevalue");
db.setObject('user:' + uid + ':someobject', {one: "thingone", two: "thingtwo"});
You just have to make sure to delete it if the user is deleted, so there's not an orphan key left behind.
db.delete('user:' + uid + ':something');
