I want to make a simulation of polymer and I want to keep density constant with increasing the simulation box to decrease the pressure of system. Is that possible?


That is not a question of the settings in an MD code. Rather you would need to invent a new kind of physics for it to be possible to change the volume yet keep the density constant.

If this is in reference to the input you posted in a different thread, then you are trying to solve the wrong problem. Your problems are much more fundamental and those cannot be solved by suppressing the symptoms.