You would have to create your own plugin to do the monitoring. The easiest would be to create a simple 'script' plugin and call a shell script that does the expiration check or returns, say, the days left on a certificate. Then you would create an alert to check this value.
The script plugin is documented here: https://docs.jboss.org/author/display/RHQ/Plugins+-+Script ... Obviously you would need some sort of wrapper script to run that would return the number of days remaining.
If you're good at writing Java, I would probably go that route instead because all the logic would be packaged into one file.