I can't find the words to explain it better, so I will give an example: Lets say that in a lucid dream there is a room with buttons that represent each emotion and I truly convince myself that if I press it I will get that emotion. When I press the Anger Button, I suddenly feel angry out of nowhere, when I press the Happiness Button I feel happy out of nowhere. I remind you that this was an example and I did not actually experience this. So, would such thing be possible in a lucid dream?
^^ Though I like to think that anything is possible in a dream, this seems unlikely.
By pushing those buttons you would probably just find yourself mimicking emotions rather than actually having them. In other words, you will simply imagine yourself angry and then do what ever is appropriate to seem angry, even though you don't actually feel angry. This imagining, I think, would be done by both the conscious and unconscious portions of your mind. But this is if you use the example you mention; what if we take it one more step?
Suppose you were able to train your dreaming mind/unconscious to produce scenarios that elicit a chosen emotion? For instance, what if you learned to incubate actual dreams that carried themes or events that would surely make you, say, angry? Though I think you could only have this occur when not lucid, because you must be surprised by events in order to have honest emotions, elicited, but you might, with practice, be able to incubate dreams that cause specific emotions before sleep. Then you might be able to go to sleep with the "button" for, say, sadness pressed, and come (NLD) dreamtime you might have a dream about the untimely death of a loved one, or perhaps a dream that removes all hope from your world...not quite the same thing, but close.
Okay I just realized before posting this that Sageous pretty much had the same idea, but oh well.
I think this would be possible but not through buttons. When lucid dreaming, we can interact with the dream world in many ways, if you were to create buttons and Push them with the expectation of feeling emotions you would surely get a different outcome if you were to feel the emotion and then focus on it rather than pushing a button.
If a dreamer was to push the 'sad' button the outcome will depend on are "conscious unconscious" (as Robert Waggoner described it) and what will unconsciously go through your mind is your memory of people pushing buttons, sometimes they work, sometimes they don't, as well as your mind suggesting that even though you might think you are in control through expectation of the dream world your mind will have trouble comprehending these emotion buttons because it simply does not make sense, pressing a button from a machine and feeling emotion is just strange, no matter how much you expect the outcome to be true since your dreaming, some way your mind will find a way to give a response that will most likely not be what your expecting.
But if you were to focus on the intent and will of feeling such emotions, not by pressing buttons or alike, I do believe you can manipulate emotions, whether that be through memories of past events that in which you have felt those emotions or perhaps by some more supernatural force that is each human emotion as a single thought-form and you can better link with this force through a altered brain state, such as lucid dreaming.
Always good to keep an open-mind.
The example provided could be realistically achieved with classical (Pavlovian) conditioning. You'd have to spend time IWL cultivating the mental triggers, of course, but it's not outside the realm of possibility...