OK, so there isn't a way to do importing, or copying from another quest, like you can normally copy maps from within a single quest.
EDIT: I have no idea why I hadn't just tried in the first place, but you can copy maps and events out of otherquests and into your quest. It works perfectly assuming your base Tileset and stuff is all the same, as a test I simply saved two slightly different variants of the same quest and then copied a few things over, it would appear you can move whole layers, whole maps, and single events without any hassle. If the files have the same name, it doesn't seem to work, but so long as they are different it is fine.