I make new land by this method :
Necessary part :
Add land in map_heights
Resize map_heights to the size op map_regions , by using the 'None' in 'Interpolation'
Copy it , make a new layer in map_regions , and paste the map_heights on the new layer .
Then , drop the opacity a bit , so you can see where you need to add land .
Delete the new layer in map_regions when you are done
Save it
Now , resize map_heights to the way it was , and save it . Delete map.rwm and map_heights.hgt (if you haven't done so already)
Optional :
Add climates in map_climates and ground types in map_ground_types .. method is the same as above