Discussion about Heroes 4.

Could you please tell me if it's possible to extract a single map from a campaign, edit it, and then insert it back in its place?

Yes, it is possible. There's a special archiver for that; you need to search for it online. I don't remember the name (something like MMH), but the version seems to be 1.37. It should be available on this website: http://handbookhmm.ru/.
I just checked the website, and it seems like this one: http://handbookhmm.ru/programs/H4_Resource_Explorer_1.4.7z, and you can use this one for the campaign: http://handbookhmm.ru/programs/h4_unpacker.7z.