Firstly this is an aircraft manufactuer question to answer.
However, in my consideration the number one priority if you find yourself in known volcanic ash (inflight symptoms) is to decrease the engine turbine temperatures. Failure to do so will cause internal damage (glassification) which is permanent. Adding bleeds without decreasing the throttle position will likely increase engine turbine temperatures.
Now if the damage is already done (engines have already stalled) then adding bleeds might help. But first of all get the hell out of the ash by decreasing throttle