The Filled Surface 
feature constructs a surface patch with any number of sides,
within a boundary defined by existing model edges, sketches, or curves,
including composite curves. You can use this feature to construct a surface
to fill a gap in a model. You can use the Filled
Surface tool in these cases:
Correct a part that does not import correctly into SolidWorks (where there are missing faces).
Fill the holes in a part used for core and cavity molding.
Construct a surface for industrial design applications.
Create a solid.
Include features as separate entities, or merge those features.
To create a filled surface:
Click Filled
Surface on the Surfaces toolbar, or click Insert,
Surface, Fill.
Set the PropertyManager options.
Click OK
.