You can transfer any existing files / folders via an ftp client such as FileZilla. Here's a rundown of how to do so:
1. Purchase a server with us, allow for the server to be setup and login to your multicraft control panel with the details provided via email.
2. Download the FileZilla client from here.
3. Use the following FTP file details to connect to your server:
Host: You can find this by clicking on the FTP File Access section of your multicraft control panel. (Note: This is not your server IP)
Username: You can find this by clicking on the FTP File Access section of your multicraft control panel. (Note: This is not your multicraft username.)
Password: This is your multicraft password.
4. Drag all of your existing files / folders from your desktop (or from where you made the backup of your server files) into the main root directory of your FTP client.
5. Drag the main .JAR file (minecraft_server.jar for vanilla and craftbukkit.jar for craftbukkit.jar) into a folder called 'jar' which you will need to create.
6. Select the JAR Name from the available drop-down menu on your multicraft control panel, relevant to the modpack you are using.
7. Start your server.
If you seek further support regarding this process, please submit a support ticket here.