It depends on how deep you want to dive - for relatively shallow submergence, one could imagine a fibreglass section of pressure hull. However, this would have some effect on radar performance, as the current equipment and control electronics will probably be optimised for thinner protection from the elements. The arrays themselves would need to be flat - although the covers could be streamlined.
TBH a better approach would probably be to stick the radar on a mast and extend it when surfaced - IIRC Oscar and Echo / Juliet cruise missile submarines have quite large electronics gear crammed into the sail. You'd need a different radar if there were less faces, though.
RP1