You state, you own a CA certificate. If that is correct, so you own a 'Certificate Authority Certificate' aka. you own a certificate enabled for signing other certificates, you can.
If you own a certificate issued by GeoTrust as a normal SSL Certificate, you can not use it to sign the SSH server certificate. Check your informations from GeoTrust for more information what kind of cert you have.
If you have the second case, consider this and this solution to possibly convert the certificates.
It is unlikely that you are in case 1. It would mean you own a trusted root or intermediate certificate trusted by your (public) clients (aka you could do the job of GeoTrust). This is only likely if you are in a closed environment (as an internal office network) and you own the certificate authority. In your question you state, you got your cert from GeoTrust, so I assume you fit within the second case.
There you may have to convert the certificates for them to have the right format, follow the links to see, if they solve your issue.