There are plenty of modded maps available on the Steam workshop for download. Here's how you can get setup and running on your server. This guide was created with the following products: Gravel Host Ark Server
1. Install the mod normally according to this guide.
2. Make note of the Map Name on the Steam workshop page for the mod. In this case, you can see it for the mod: Olympus.
3. Open the Startup tab, and rename the field Startup Map to the corresponding modded map name
4. Save the file and restart the server.
That's it, you've now successfully changed your ARK map.