Adding more packages to a wstool generated workspace

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I have made a source build of hydro using the instructions here (substituting hydro for groovy):

http://wiki.ros.org/groovy/Installation/Source

I followed REP 131 and only made a rosinstall including viz.

However, now I would like to add perception to the workspace. So I created a new rosinstall as such:

~/ros_catkin_ws$ rosinstall_generator perception --rosdistro hydro --deps --wet-only > hydro-perception-wet.rosinstall

~/ros_catkin_ws$ wstool init -j8 src hydro-perception-wet.rosinstall

But when I tried that I got the following error:

Error: There already is a workspace config file .rosinstall at "src". Use wstool install/modify.

I tried looking for documentation of wstool here:

http://wiki.ros.org/wstool

But the documentation is sparse. When I type wstool help, I don't see either install or modify as options.

Is there a way to build upon a previous wstool-created workspace or do I need to make a ros_catkin_ws_2 or something like that?

Any help would be appreciated!