Well, this is not pumping, but it might be much more efficient: As I understand, the polar ice caps are in an equilibrium between snowfall and runoff. If you could somehow wall in a large portion of polar ice, such that it cannot flow away, it might rise to a much higher level and sequester enough water to make a difference in sea levels. A super-large version of a hydroelectric dam, in effect, for ice.

It might also help to have a very high wall around the patch to keep air from circulating, keeping the cold polar air where it is and reduce evaporation/sublimation.