No idea - Begonia sutherlandii is a tender tuberous perennial - methods of propagation are leaf or stem tip cuttings, or seed, never heard of 'harvesting corms' off them.

The baby corms are best stored in a cool, dry & dark place. I simply wrap them in newspaper, in a cardboard box and store in my basement. Around 40F. However, I believe these are hardy through zone 7, or 2 degrees Fahrenheit. You can probably leave them in the ground or plant them out in the Spring.

the RHS says hardy 2 ie frost tender and will cope with temp down 1-5degrees C.
having said that I buried corms deep under the beech tree and left them for years. I lost them in the harsh winter 2010/11.

never known them produce tubers. lots of seed pods though.

I'd alos keep them dry and frost free until spring then grow them on. it will be a few years before they are flowering size though.

I've just left my plant on the ground where it's been growing for the last four years, so it's gradually getting bigger.