I'm thinking it could be done but you'd essentially have to edit all of England and Northern Ireland's domestic system as one file. That's going to be a pretty big undertaking.
The way I'd probably do it, is move all the clubs in England, Wales, NI and Scotland to UK or Great Britain, run the various leagues as regional with promotion finally into the vanarama. It would be a big undertaking but it's probably the most efficient way to do it. I'm not sure off the top of my head how this would impact the domestic structures or nature of the individual nations though, nor things like salary and sponsorships,