Why does the air domain end at the flat ends of the wing?  Is this wing to be tested in a wind tunnel with walls touching the left and right faces of the wing?  That is what you have.  Or is the wing to be tested in a larger volume of air that extends beyond the left and right faces of the wing?  This is what I would expect you would want to test.  A lot of interesting flow phenomina occur beyond the tip of a wing.  Make the width of the block at least ten times wider than the width of the wing. That is a minimum to study flow over a wing, a width twenty times the wing width is better.

You do not need to convert the wing to a single surface.  The mathematics used to represent the shape do not allow a single surface to represent that shape. There will be no problem meshing the air with a solid wing made up of multiple surfaces.  If any problem is found with the element quality during meshing, it will be improved by dividing the air domain up into pieces, possibly dividing the wing into pieces, adding to the total number of surfaces on the wing.

The SpaceClaim document was a solid (filled) body. There is no need to export a STEP file. DesignModeler can open a SpaceClaim file.