My bad, the language I chose was ambiguous.
I meant to ask where to bitGOLD, bitSILVER etc come from, in the sense of what's stopping anyone from adding bitDRK or bitDOGE. Are bitGOLD, bitSILVER, bitBTC (and so on) built-in somehow, and can new ones be added?
I am assuming the answer (to the 2nd question) is yes -- it's the how that perplexes me!