This mod will allow you to add new things to already explored areas. I used this to add Tar Pits and Onions to the map we've been using, it took some doing, and you have to remember to force a save after adding the new content, but it seems to have worked very well on our map.

Remember to backup your map before trying this just in case!

https://www.nexusmods.com/valheim/mods/1485?tab=description