No Automated Mod Manager
7 Days to Die doesn't have a Workshop browser or any automated mod installer on Spawnly, and there's no confirmed, documented folder convention for this server build that we can point you to with confidence.
What You Can Do
The Files tab (File Manager) gives you full access to the server's files, so it's technically possible to add mod files there yourself. But since we haven't verified a working mod-install path for this specific server build, treat this as an advanced, experimental option rather than a supported workflow — don't expect a guaranteed procedure the way you'd get with a Workshop-enabled game.
If you do experiment with mod files, take a backup first so you can roll back cleanly if something breaks the server.
Next Steps
Take a backup before touching any files — see Managing Backups & Restoring Your Server. If you're still setting up your server, Getting Started with 7 Days to Die Server Hosting covers the basics.