If you just have 1 website, you can install JMS on it and convert it in a master.
After that, when you have a master, you can create other websites.
If, like recommended in tutorial video 0.b (slide 21-24), you want to have a master beside your current website, you can effectivelly move the current site as a non public subdomain that you will use to rebuild your current domain.
If you need backup/restore, we publish in the faq a manual procedure and also provde a shell script that you can be used to perform such backup.
The restore may require a "MySQL root" login.
www.jms2win.com/en/faq/faq-joomla-multi-sites#cat-268
www.jms2win.com/en/component/docman/doc_...weekly-backup-script