Changing existing NPC dialogue

[SterkOks]

Is it possible for me to change what an NPC says? I'm currently making a silly mod for a friend where Sheogorath is sitting in a room. All I want him to say is "Do you mind? I'm doing the fishstick..." The reason I want him to say this is because he's supposed to be 'meditating,' or whatever you wish to call it. I can find it in his dialogue box if I double click on the actor in the render window, under Misc., however, it won't let me move or edit these. Is it possible to make one specific line of dialogue active while the rest are never said? Thank you for the help!