I guess a more accurate question would have been, how can the tokens be "extracted?" And the answer to that is, someone would have to create the ability to do it?
Basically correct. But (and maybe I'm being a big pedantic here, apologies) in cryptocurrencies there's no "extract." If you deposit STEEM on Bittrex, your STEEM never leave Steem - they're sitting in @bittrex's wallet the whole time. They're still part of Steem.
In Pocket, I solved what you're calling the "extract" function by allowing people to include a memo with their pocketsend.
So to get listed on an exchange, some exchange will have to list them - and they'll have to create the system that associated pocketsend memos with exchange accounts, etc. I'll talk more about this after Genesis ends in 12 days; I'm not supporting listing until then. If someone wanted to do it themselves, of course, I couldn't stop them.