Off-hand, the simplest workable scenario would be in a case where the sim is already going to get a moodlet from something. Like say they normally get uncomfortable when they're tired. You can use buff_replacements as part of a trait to replace the normal uncomfortable buff with a buff of your own, such as a sad moodlet.
♫ Improvising into a corner ♫
♫ No rhyme or rhythm in this verse cause I'm in a corner ♫