What code to add?
Since every module is doing something differently, what comes in on the $_data array will be different in each case.
So what you need to add or change will be different in each case.
Probably the first step is to check out the modules docs to see how it wants additions added (if its specifically asking for additions like the jrEmbed module does).
Otherwise look at other modules also listening for the same event and see how they are setup.
The way I like to do it is to setup my listener, and create the function that fires with just an empty structure and return the $_data.
Then set a break point in my debugger on the
and make the function fire.
This will show me what I have to work with.
(see 'setting up a debugging environment' for more details)