Any blank heightmap that is in the correct format and size will work in the game, whether you make it by hand, generate it in a third party generator like KingGen or Teragon or World Machine, or get it online. I have even used online heightmaps of Mars and the moon that were not created with games in mind and they work fine.
You should only have potential problems if you have other things included that are not done correctly for the game - POI, roads, water, etc. But a heightmap is easy. It just needs to be the correct format and size.