A couple of things come to mind - hang some weight from the centre post so that the tripod doesn't move, or throw some weight on one or more of the legs where they meet the ground to keep them from moving. I would also shop around in my camera stores to see if I could find something that fits over the existing feet that maybe had spikes in it.
Alternatively, you can build something yourself - I would get some short lengths of ABS pipe, what ever diameter will fit snugly over the bottom of your tripod legs, you probably only need pieces about 10 cm long. Get a cap for each piece, drive a small nail or screw through the end of the cap so that it sticks out the top of the cap, put some glue on it to keep it in place and fit the cap over one end of each peice of pipe, cut the nail or screw to say 2 or 3 cm long and then stick the end of each tripod leg into the pipe and voila, you have something that will stick into the snow and ice. Haven't worked out how you keep these pieces of pipe on the end of the tripod legs, but I am sure that there is a solution for that too. My 0.02¢ FWIW.
Cheers,
WesternGuy
Afterthought: If you make the pipe pieces about 30 to 50 cm long, then you may be able to use some velcro tape, or gaffers tape to stick the legs onto the tripod legs as an interim solution - don't know - just a thought.