Whow do you add an existing digital ocean server to forge? Let's say I have a DB server that I created some time ago and do not want to spin a new one just for sake of it.
Well, I don't think it's possible right now. Forge needs to install some items on the server to make deployments work correctly. It also needs to setup the correct directory structure, nginx config, etc. Your current setup is probably different from that from Forge and that makes it impossible right now.
My suggestion would be to create a new server and simply migrate everything to the new server ;) That's a max downtime of 5 min!