You can add a custom world easily using SFTP. All you will need is to be logged in on your SFTP client.

If you are downloading from a singleplayer world, you must rename the world to world before downloading.
You will need to use SFTP to edit world files.

First, connect to your server via SFTP.

Extract the world files from the .zip file. You can do this by extracting the files, or by opening the folder.

Move the world file to the server's main directory using your SFTP client.

Once you have moved the world to your server, ensure that level-name=world in your server.properties file.



Make sure that the worlds are for the version that your server is, otherwise the world can corrupt and the server may not start.