It is possible to install the certificate, previously issued outside the 'Namecheap SSL' plugin.
For this, you will need to reissue the certificate within our plugin. This will create a new RSA Private key on the server, as the original one cannot be recognized by the plugin.
You can refer here for reissue instructions.
Once done, the certificate will be installed on the server, and you'll be able to check it using this tool