One of the good things about open-source projects is that! Of course you can improve it. If you want to extend jBPM5 first take a look at its code here: https://github.com/droolsjbpm/jbpm
Then you can join IRC channel #jbpm in irc.codehaus.org server. There you can find krisv, which is the lead developer of jBPM5 and tsurdilo, "the tooling" guy. So feel free to enter the channel and start asking.