If you're Ukrainian you would sell your Hryvnia either on the open market or to the government, to buy dollars or euros and then import goods. This is why countries can implement currency controls and try to set exchange rates.

It's less that IS has its own currency than that they're likely to ban the use of anything else. This gives them the sole and exclusive control over other currencies, and given that trade in and out of their territory is limited to smuggling through increasingly-limited border crossings, full control of all trade in general. It gives them a level of control over the economic life that governments like Ukraine don't have. If the hryvnia collapsed, they would start crucifying people for switching to dollars and circumventing government oversight, particularly exorbitant taxation. I wouldn't put that past IS. It's a bit of a unique situation because IS has so much experience in smuggling and illicit economic activity itself.