I have seen a quick fix by adding a power strap or ski strap between your top two buckles of your boots to hold the gaiter of the snow pant up a little and this raises up the pants themselves. Also, suspenders can help, but after a while they just give you a wedgie (they can only go so far up, right?).  It isn't great, but an alteration on waterproof equipment is no easy ask as @REI-JohnJ points out.