I managed to install JBPM into my own eclipse by editing the install scripts that came with the JBPM installer. It doesn't take much.
It has been a few weeks since I did this so my memory is fuzzy.
I dug into the JBPM installer and found a proeprties file and a build.xml file. I modified the two files so they would install the drools flow and jbpm plugins into MY eclipse rather than the one the installer downloads. My specific order was I ran the script through unchanged once just to make sure all of the pieces parts downloaded into the expected locations then I modified the two files so they will install the plugins into my eclipse.
I can dig up the details Monday if somebody wants more details about what I did specifically. If you understand Ant it only takes about 5 minutes of work. Even if you don't understand Ant it isn't a very mysterious process. The install scripts are done well enough that guessing isn't necesary.