Ubuntu - How to trust self-signed certificate with Windows GPO?

I’m running Openfire on Ubuntu. I recently introduced a number of Windows 7 clients into my environment and Spark works sporadically on them, so I had to install Psi instead. I receive a pop-up warning me of an untrusted SSL certificate when logging into Psi, since my SSL certificate on the Openfire server is self-signed. I should be able to click a button on the warning to make it go away, but it’s a little buggy, and doesn’t always make it go away permanently.

On Ubuntu, how can I export the self-signed SSL certificate and import it into a Windows GPO so my Windows 7 clients will trust the certificate and stop issueing the pop-up?