Hi Marco, I agree with what Chipp wrote above, for things that are kind of organic smooth and blobby all over, usually a sub-d polygon modeling program is better suited than a CAD program like MoI.

For trying it in MoI maybe try lofting it in the other direction though, I've attached an example where I drew in some lines from the side and projected them onto your shape to get some curves, and then did curve blends to make them closed, then used Loft with Loft Style = "loose" through those cross sections.

- Michael