No, it is not possible to have the administration functions separate from the site. You can purchase an SSL certificate and set it up on your site. Once configured for SSL, mojoPortal will automatically use SSL for all Admin pages.
You could also use the IP Address and Domain Restrictions feature in IIS to limit access to the /Admin directory to your IP address but you only have this option if you manage your own server or have a host that is willing to set it up for you. I do not recommend this configuration unless you absolutely know how to configure it and you have thoroughly tested it in a dev environment.