Yes, it's possible. You need to write a custom plugin that "extends" the jmx plugin. Chances are, you may not even have to write any Java code in the custom plugin, but only define metrics and operations in the plugin's XML descriptor. For details on how to write a plugin, see:

http://jopr.org/confluence/display/RHQ/Writing+Custom+Plugins

and:

http://support.rhq-project.org/display/RHQ/Plugins+-+Demos

I'd recommend reading this blog first:

http://management-platform.blogspot.com/2008/11/monitoring-custom-jmx-mbeans-with-jopr.html

then you can delve into the custom plugin docs further.