The internal power supply is not connected to USB like on the MacBooks. So at minimum you would need to have a inverter inline to convert DC to AC.
The typical power draw is quite high (since it has more cooling and no battery restriction vs a MacBook). There are not many cheap (or even expensive) power banks that can supply the full power needed and the iMac doesn't have an internal battery to make up the difference.
I imagine the closest you can get is getting a Uninterruptible Power Supply, as it gives you a regular AC socket and can tell the computer to turn off when it runs low.