How hard/possible it is for an engineer to make this happen?
Well they are manufactured by machines, but prototypes or even small production lots could be put together by hand. 

A competent power supply engineer with experience in offline converters could design a custom converter for you, but then there are a bunch of regulatory requirements and tests that you have to do which may vary from country to country.  So it can get expensive depending on which and how many countries you want to sell it in.  Certainly possible though.

Any way of procuring an already built and tested supply is likely to be cheaper, even if it means flying to wherever they are built to make some sort of import deal.