A down outer garment is obviously one of the warmest available. It will not impede the wicking of your wool underwear per se, but should not be expected to continue the wicking process. Ideally, moisture will begin evaporating through the down garment. That is why the shell should be breathable. The worst case is that the down begins to get wet as it then loses its insulation quality. If it is cold and dry, down is the best. If it is going to be wet, something else might be better. In addition, most shells for down garments are pretty wind resistant because the weave has to be tight enough to keep the down from escaping. If the down baffles are not overlapped one can sometimes feel the wind through the stitching.
