Set different world generation around an existing map with structures?
Hey, so I recently made a map for a build with a friend on an Aternos server. The map I made in Worldpainter is about 2,700x2,000 blocks and a mountainous/plains area. The problem is that when I export it to a Minecraft map, an ocean with islands generates around the existing Worldpainter map. Now that we have already built structures in this map, I'd like to change the surrounding generation to also be a plains biome. I have tried setting a seed in the Aternos server settings before joining for the first time, but this didn't help, the same ocean still generated around the map.
Is there any way to force only a certain biome to generate around the map or alternatively just generate a plains world and paste the map I already made into it?
I'd greatly appreciate any help/tips, thanks!