Oh, we have been asking for this since Beta on the PC.
It's a GUI, there is no reason we can't click and slide around the character slots on that screen. Even if we have to give them a "Priority Number" so they go in the order we want, there is no reason this can not be done.
However since, AFAIK, no one has a made a mod to do just this, it might be something in the code that needs an overhaul.
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