I assume that you want to write code to thicken a bspline surface because you don't have a license for the Surfacing module, because that module has a Thicken command.
Since a Bspline face is generally "curvey", there is no single normal direction for it. The normal direction will, in general, vary across the surface. The command sd-inq-face-pnt can be used to determine the normal at a point on the surface. You can use sd-get-pnt-on-face to find a point on the face, although you will probably need to use sd-proj-pnt-on-face (along with sd-face-relax-pnt)to convert that point into something that can be used for sd-inq-face-pnt.