Mixing deterministic and stochastic shocks

Dear all,

I am trying to simulate a model where I would like to add an expected productivity shock which in turn proves to be unrealistic.

For expected productivity shock I wrote the equations as following

EE_A = (rho_a)*EE_A(-1)+EE_exp(-4)+E_A;

But I could not figure out how I can define another shock which offsets the effect of expectation shock. In other words, I would like to give a shock to the model in the 5th period which equals to the negative of the expectation shock so that productivity does not increase in contrast to expectations.

I would really appreciate if anyone could help me.