I think the only option is to get a certified certificate from a 3rd party authority. This would allow every computer to automatically trust the cert if that is what you are aiming for.
To upload the 3rd party cert In the GUI, go to:
Management > HTTP
This is where the HTTPS certificate is managed.
If you have a 3rd party certificate from a CA, I believe you click Delete Certificate, then click the Download SSL Certificate and fill in the tftp details.
It required a reboot for the certificate to take effect.
This config guide explains in more detail the installation process... http://www.cisco.com/en/US/docs/wireless/controller/5.1/configuration/guide/c51intf.html#wp1101169
