If they were standard plastic varifocal lenses there would be no problem. However, high index material can be tinted but not very dark (probably not dark enough to wear as sunglasses) and if it has a anti-reflection (ultraclear) coating or a hard coat then it cannot be tinted at all. So you may have to purchase new lenses.
Yes, varifocal high index lenses can be tinted as sunglasses,high index sunglasses are often offered in tinted colors.tinted lenses are best for bright and overcast outdoor conditions. high index sunglasses are more scratch resistant than polycarbonate, but they are more reflective. It is a good idea to get anti-reflective coating to reduce glare, especially if they'll be used for work or driving.