When are you doing this?
They can definitely sell it at Nottingham, and they should if asked. However I am unsure about on board. They should sell it but we've had some reports of some machines having the rover/ranger function disabled. I doubt you'll have a problem on Northern though.
The cheapest way to get to the area from Nottingham is probably with a Nottingham to Barton on Humber ticket (£15.30 is the full fare) and then the bus. This ticket is a lot cheaper than Nottingham - Sheffield plus Sheffield - Gilberdyke tickets. In fact, it ranks up there with Ashton to Donny in the list of great value, useful tickets.
If it's a weekday it may be different though as you'd have peak fares to consider.