To be able to apply the free SSL certificates your site(s) need to use the 20i nameservers.
This is because the SSL certificates are installed locally to the 20i platform.
You can however use our Simple SSL which can be purchased from within your your account: https://my.20i.com/services/order-ssl
If you already have an external SSL you can install this too. To do this you'll just need the Certificate, Private Key and CA Bundle / Intermediate Certificate. Then simply login to your My20i account and head to the SSL/TLS section within the hosting package and paste in the three sections mentioned above.