Add the surface pressure to a 3D grid (LAT, LON ,P)

I have defined my 3D grid in longitude,latitude and pressure, because I'm trying to plot the residual circulation of the atmosphere, and my vector pressure has values [10,1000] hPa, lon is [-180:180] and lat is [20,50], my grid resolution is 1º. My problem is that now I need into account the surface pressure for each latitude and longitude grid point, I have download tha surface pressure data in each gridpoint but how can I introduce it in my initial grid?

Thanks.