I'm curious too. AFAIK, the transponder bits are mostly identical, only the mechanical part is matched to the car. There is a unique electronic ID#, the EWS module uses it to identify the key, but doesn't care what the number is. The transponders are synched to the car during the initialisation procedure, that's why you have to synch all keys at the same time. You don't need a matched set per se before initialisation. So if they somehow sourced a proper transponder key head (BMW claims they are the only possible provider), then cut the key shaft, then it should synch to the car along with the current keys without issue.
Don't really know for sure, that's why I'm curious about what they really do too.
BTW, if you need to code more than six replacement keys to the car (10 total), you can do so by replacing the EWS module, but only BMW has the code that needs to be inserted to match it to the existing ECU. Compared to the key cost, the EWS is a bargain, so don't worry too much about the six key limit.