Hi Riccardo,
Yes - every element in the chart support transparency trough the fill style associated with it:
constLine.FillStyle = new NColorFillStyle(new NArgbColor(125, Color.SteelBlue));
You can also take a look at the following example shipped with the control:
All Examples\Axes\Attributes\Constlines 3D
Hope this helps - let me know if you have any questions...
Best regards,
Bob