The answer is simple (to the original question, in the title): If you want a SD-slot, get an eReader that has it. Thanks <insert your pet god here> there are plenty of choices, today.
If you're not happy that eReader A has this SD but has some disadvantage but eReader B is better but it has no SD-slot, I may ask you, are you happy with your house, your car, your wife, your president/king
? Compromises are made every day, for each little thing one has or buy.