Please check if you can edit the library when accessing from Preferences:
Window->Preferences->Java->Build Path->User Libraries
(If I remember the path correctly for windows )
Turns out that the jBPM library is not a user defined library. I found it's settings here (see screenshot) but there are no options to enable/disable modification of things like javadoc locations.
I suppose I could create my own version (using user library) but it's not really worth all that trouble.
I'm not a strong eclipse user, but if I don't remember wrong with m2eclipse you should be able to see the project dependencies and download all the javadocs and source packages related with your project dependencies as part of the plugin functionality
I know this thread is 2 years old, but people will find it if they google "eclipse attach source to non modifiable", and will end up here. The problem is that Eclipse has all the buttons that are normally used to attach source to a project disabled...so it's unclear how to attach the source for the "jBPM Library". Sorry, I have a duplicate of this thread that I also posted myself onto this forum.
UPDATE: I posted the solution on this thread. Basically you have to set your source folder as the "jBPM Runtime" in one of the jBPM dialogs in eclipse.