If anyone every looks at this I figured it out and it's super easy.
SyntaxEditor Code Snippet
Dim oSketch As Inventor.PlanarSketch
oSketch = oPartDef.Sketches.Add(oPartDef.WorkPlanes(2))
' Project 3D Spline
Dim oSpline As SketchSpline
oSpline = oSketch.AddByProjectingEntity(Spline3D)
Where Spline3D is the spline created in the 3D sketch. Doing it this way allows you to constrain other 2d sketch objects to the projected spline.