At first order, you have to change line 83 of the simult.m in the Dynare/matlab folder, which reads
DynareResults.exo_simul(:,i_exo_var) = randn(nxs,DynareOptions.periods)'*chol_S;
to something like
DynareResults.exo_simul(:,i_exo_var) = 2*(rand(nxs,DynareOptions.periods)-0.5);
Due to certainty equivalence, it does not matter that the shocks you are drawing now have a different standard deviation than is used for computing the decision rules. At higher order, this approach would be wrong.