I understand your problem, but I do think that for now there is no easy solution ahead.
The best I think of what can be done today is what I advise above.
In the future, who knows? Sony can upgrade their firmware with ePUB as they have promised that some time ago... or Cybook could do it too, as they have mentioned it several times... or... hell with this "so much" formats around... is very dificult to see what the future will bring.
Best,
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