> Is it not possible to do one of these two?

I think they are asking for the proper way of modifying the script
that generates barcodes in koha for NEW items, preserving the
semantics of the current system they're using.

I don't think so, based on:

Post migration we will generate & print barcodes from Koha; so we dont have to emulate this logic in the barcode generation for Koha

Though I did mention they may want to seed a valid new max value for their intended barcode scheme once everything is loaded. I.E., their values are not numerical, so they can't use incremental, and they'll need to use something else.