I believe Compress does not do calculations for standard pipe fittings. You can model your pipe cap as a 2:1 SE head to graphically show it; but I would not rely on Compress' calculation output for that particular component.
If your vessel code is ASME VIII-1 and your pipe cap is an ASME B16.9 fitting, refer to UG-44.
Be very careful in your documentation of this calculation. Should an ASME team leader find a pipe cap calculated as an ellipsoidal head during a review you will get some grief. UG-44 is quite specific as to how it is calculated.