Yes: if you have an existing external or third-party SSL certificate you can install this at 20i.
Your certificate provider will need to provide the following sections for us to be able to install the SSL certificate:
These details will need to be entered within the SSL/TLS section within a 20i hosting package: Manage Hosting -> [select package] -> SSL/TLS.
Please also note that SSL certificates can take up to 30 minutes to install on to the web server. If your A records are then pointing to the IP address of the hosting package then your site should show as secure.