How to add a spacer(similar to tubesheet) between two body flanges?

In a pressure vessel I have a spacer Similar to tubesheet. I have a Integral flange with hub + Metal ring gasket + Spacer + Metal ring Gasket +  Integral flange with hub. This is a simple question in Heat exchangers, but I think it isn't possible do it in Pressure Vessel package.

Does anyone know how to do it?