I guess their employees will have to use Bitcoin ATM's or buy Bitcoin directly from other people, or use some proxies (like their friends of relatives), because for banks it shouldn't be hard to check if someone is depositing fiat to exchanges. And this is how Bitcoin ban or restriction might work - governments will cut fiat systems from cryptocurrencies, so the market will have to go underground, and we'll have obstacles like lower volumes, risks of p2p trading.