While humanizing an individual tracks tempo automatically isn't really an option in LMMS currently, one can mimic other aspects of humanization with LFO controllers and/or automation tracks. For example one could humanize impact volume by connecting a LFO controler to it. To add a bit more randomness one could use a second LFO to control the base of the fist LFO. For even more variation one could add a RC Lowpass filter 12x to something like a drum, and then either assign another pair of LFO controllers to the cutoff, or assign the same ones that are assigned to the volume control. If you don't mind a little manual work you can add an automation track that controls the base value of the second LFO controller to emphasize key points.