How to convert the B-spline surface to Smart solid

Hi all,

I'm using the OpenBuildings Dsigner.

I face some problems:

1. When I sweep the scupper drain profile, it will become the B-spline surface. It is unable to convert to solid. 

2. I know there is the way to convert to solid which is by "Stitch". However, the sweep surface is unable to select when I want to stitch it. 

Does anyone know how to make the B-spline surface to Smart solid ???? And, how to use the stitch function ???

  

This is the B-spline surface model.

Thank you for your big help in advance.