USB powered "fake" battery pack adapter?

[pier4r]

So I tried to search a bit but my question may be a bit too fancy.

I was wondering whether it is possible to have battery compartment with 4 AAA (or 3 or 2, etc..) faked batteries that in reality distribute the power from a usb charger.
Of course a problem could be the voltage. AS the usb is 5V, the batteries are normally ~ 1.2 V .

Anyway, do you know anything similar? I ask this because it is a possibility for me to use more freely calculators like the ti89 , the 1st gen nspire, 49g and others. So far the usb powered systems are the casio with "usb power graphic" label (9860g, 9860gII, various fx-cg10,20,50, etc..), the 50g, the dm42, the prime, numworks and the nspire CX. The list is surely not short, but there are different old models that would be cool to have usb powered as well.

For example the relatively cheap (sometimes) and large voyage 200 and so on.