Adding a trigger to a state while in the same state

Although I agree with Phil about this use of states, and feel that states in general tend to be a bit iffy, I understand that sometimes you really just need an answer to the question. Putting a bunch of objects and triggers on states makes then difficult to understand, edit, and maintain. It's generally better to make states more visual and trigger actions separately, and externally.

That said, if you really need to edit a state as you describe then try this:

• Save a backup of your project just in case
• Make a new empty layer on your base slide
• Return to the base slide layer
• Select the object you're interested in
• Switch to the Sate tab in the timeline
• Click Edit State
• Go to the state in which you wish to add a trigger (click on it to select)
• Click in the main slide window
• Type CTRL-A to select all objects within your state
• Type CTRL-X to cut the objects out of the state
• Click Done Editing in the State tab
• Return to the Timeline view
• Click on the new blank base slide layer you created in step 1
• Click somewhere in the main slide window
• Type CTRL-SHIFT-D to duplicate all of the objects you cut out, in their original positions
• Do whatever editing you want (including adding triggers)
• When finished, reverse the above steps. In other words: 
• In the layer, select the main slide window, CTRL-A, CTRL-X
• Return to base slide, select the original slide object, go to State tab
• Edit State, select the now empty state you wanted to edit
• Click in main slide window, CTRL-SHIFT-D, click Done Editing State

Your state should now show as it was (or however you changed it), with any added triggers visible. Make sure to test the functionality to ensure nothing broke during the transfer process.

I tried it with your slide, adding a variable change trigger on media complete (within the state) and a text box displaying the T/F variable value on the base layer to test it. When the media on the state completed, the variable changed as expected.

If you need to make complex states for some reason, it is often easier to manage if you do it outside of the states, and then paste it as a whole into your state when it's ready.