Stellar allows you to send and receive any type of currency. So it just means allowing people to deposit and withdrawal via Stellar in addition to whatever other deposit and withdrawal methods the exchange supports.
For example now on Kraken you can send a Sepa payment to deposit Euros. They could also accept Euros sent in via Stellar.
As an exchange operator I need banking connections to get a fiat backend. I don't know of any bank which supports Ripple/Stellar - only a few banks in the world deal with crypto at all. I guess I'll never understand this concept. To move fiat in the banking network one needs to be a bank.