The only way to get back directional control is un-reverse and forward idle.
In fact this becomes a very skilful and tricky manoeuvre when practiced in a simulator. The Boeing 737 FCTM advice is to first stop any braking - whether autobrakes or manual. Then select idle reverse - (not forward idle) while keeping in mind that it takes several seconds to actually reach idle N1 from high power reverse. Once the aircraft is under control using flight controls and aligned with the runway direction, re-introduce braking and symmetric reverse thrust. A significant amount of runway is used while all this is going on.
It takes considerable time to wind up the engines from ground idle reverse to full reverse and by the time the engines reach full reverse it is not very effective anyway in the low speed below 80 knot range. In the simulator we often noted difficulty experienced by the PF is locating the reverse idle detent by feel; especially if the idle reverse detent is sometimes ill-defined. In order to quickly reach idle reverse N1, the levers must be moved rapidly (not slowly) down to the reverse detent There is no time to look down to where you think the detent is, as all concentration is outside the cockpit. If the pilot misses the detent at first go and inadvertently goes into forward thrust, this can exacerbate a sideways slide especially if one engine goes to idle reverse while the other goes into forward thrust while winding down towards the idle reverse detent.
Crews that do not have regular practice in the simulator on slippery runway crosswind operations, may be unpleasantly surprised when it happens for real - especially to have the presence of mind to first completely release all braking action which would seen contrary to commonsense handling on a slippery runway. There are several ways to release all braking dependent on aircraft type and this needs to be briefed if a slippery runway crosswind landing is contemplated. From experience in the simulator it takes at least five practice runs (touch-down and roll-outs) before confidence and competence are attained. The correct sequence of actions is vital.