1) a way to speed up the process is copying all heroes to the campaign object data (in a new campaign window), then opening the new map, then copying every hero in the campaign window to the new map. Saves you some time from switching through both maps...
2) Jass Newgen pack (check wc3c for a download mirror) has an object merger. No clue how it works (never actually needed it), but I guess it could be helpful for you.