I think that the best person to review this mappers is @afirlejczyk

Shouldn't there be at least some kind of possibility to then add custom attributes to the cast maps? Or is this then left to the user to override?

Is there work in progress?
I ran into this problem with german postcodes which could contain a leading zero.
It obviously won't work without a specific mapping.

Shouldn't there be at least some kind of possibility to then add custom attributes to the cast maps? Or is this then left to the user to override?

You could easily use the Magento module system to overwrite a mapper classes you need to extend.

@sandermangel @nhp I added an updated version of this PR (#49) with an approach to extend the mapping without class overwrites. It works like in the DivanteLtd/magento1-vsbridge-indexer and its custom data-sources.