Any token on the Steem blockchain can theoretically be added to an exchange; see the Steem Blockchain Dollar. That being said, exchanges will only want to host tokens that have reached a certain amount of value and volume. All SMTs will be immediately liquid on the internal exchange and SMT creators will be able to back their token with STEEM which will create liquidity for their token using the automated market maker logic baked into the SMT protocol. But even once an SMT is on external exchanges, it will still be on the internal exchange just like SBD.