In AI*Environment / ICEMCFD 10.0: 1. Firstly we need boundary curves. These can be obtained by running build topology (Geometry > Repair Geometry > Build Diagnostic Topology) on the entire model (Method = All parts), alternatively you can run build topology on just the surface you wish to convert by setting Method = Selection. 2. Go to Geometry > Create / Modify surface > Merge/Reapproximate surfaces and choose Method = Reapproximate selected surfaces. Choose the facetted surface and define an appropriate tolerance (default = 0.1). Hit Apply or Ok to convert the surface to b-spline format.