Client certificate help

I setup a Fortigate 60E with VPN access last week and using a cheap ssl certificate we had for a domain of ours we didn't use. I'm trying to work out how i create individual client certificates from this? I have it working where the certificate is install don fortigate and using ssl but they dont need client certificate however I'm looking to add that extra security.

The certificate i have comes with a .crt, .p7b, and ca-bundle file. I understand each client using the vpn remotely must have a client certificate that checks with the certificate installed on the fortigate.

Can i do this using open-ssl or is that not as secure as a paid certificate?