How can deal inflation and price since prices donot has S.S

I do not know how to deal with Price and Inflation in mod file:
pi = P/P(-1);
if inflation pi has steady state 1.005 for quarterly data, and Price does not have a steady state at all. I am not sure it is work in dynare or not if I put it in my mod file?
I want to recover the IRF for price level after a monetary policy shock. So I need to have price level input.

In many models (e.g. interest rate rule) the price level is non-stationary, so you cannot have prices in the model. They will not return to a steady state. I am not sure what you want to do, but you can recover the IRF for prices from the inflation rates by noting the definition of prices. We have that
This implies that
If you assume that P(0)=1, you can use the IRF for inflation to manually recover the price level. The procedure is similar to recovering the IRFs of nonstationary output after a permanent technology shock as shown in the Aguiar/Gopinath (2007) mod-file on my homepage.

Thank you very much!
Is there any way to deal with non-stationary variables in Dynare?
Is Unit_root_var command still available?

What you mean with “deal with non-stationary variables”? For stoch_simul, unit roots are no problem.