Hello,
Could you be a little more specific? I mean what is the pressure you want to prescribe? If it is the dynamic pressure the pressure gradient is assumed to be zero at the boundaries, it is not impossible to change this as we do it for friction (in this case it is the velocity gradient) but it is a bit of programming. If it is the atmospheric pressure it would be better to give an atmospheric pressure that gives the requested gradient, and the program will compute its gradient.
With best regards,
Jean-Michel Hervouet