Mike 2 has so many morphs already I would have thought you could have done this, so I've just tried. I found using a negative value on 'open mouth' (way down at the bottom) keeps the teeth together and 'smile left' and 'right' brought a grin in, you can adjust the lips too to make him bare his teeth more. Using a magnet on his lips alone is fraught with difficulties due to the proximity of the teeth and the curve of the face so if you can't get satisfactory results using the existing morphs (and remember you can always spawn a new morph from your adjustments by selecting that option from the object menu) then exporting the head as an object (remember to select 'as morph target, no world transformations) and make your adjustments to the polygons individually...