Hello.
Late reply…. But You can install NPM - Nginx Proxy Manager. And on tab “SSL certifcates” be aware, there are two buttons “Add SSL certificates”- both open different dialogs (UI/UX facepalm). Right dialog is opened from top right button. Click on it and see, there is possible of upload Custom SSL certificate. You can generate it using openssl utility. And upload it to NPM using this dialog.