good question, I’ll try to answer:
Harmonica manufacturers do not build or market full custom harmonicas. The procedure of fully customizing a stock harmonica cannot be conducted on an industrial scale. There are only a handful of players in the world who are able to perform this extremely exacting work, which why the list of Hohner Affiliated Customizers is necessarily short and why their instruments command such high prices. We only accept one new HAC into the program per year and there are no plans at present to increase this number. There are no formal classes available and all the HACs are self-taught. As I mentioned, they are also all master players, as it isn’t possible to do this work otherwise.
Hohner initiated a revolutionary move by establishing an independently verified HAC certification process and entering into an exclusive cooperation with the best of these craftsmen. This provides a guarantee that anyone who is permitted to use the HAC logo is delivering work of an exceptionally high standard and can legitimately claim to be building the finest harmonicas available.
I’m sorry to disappoint you, but there’s no established way of learning how to do this. Many customizers regard the highly skilled techniques they employ as their trade secrets and often aren’t willing to share them. You may find some information about customizing techniques online.
The Hohner Service Department does however offer training to become a Certified Hohner Service Technician. This takes place at company headquarters in Trossingen, so that the Head of Service can personally assess the candidate’s abilities and workmanship. Certified Service Technicians conduct all manner of repairs on Hohner instruments and may also offer some customizing services, but they are not Hohner Affiliated Customizers.
I hope this makes things a little clearer,