Creating a solid from a shell?

Hi everyone,

I work in aviation and have recently found myself having to work with solidworks. And now i have crossed the following dilemma.

I have a 4mm plane fuselage shell, created from 2 sketches and some pretty complex 3d sketch curves. I would like to get a solid of the space contained inside this shell.

My first thought was to offset the sketches to make a new loft, but modifying the 3d sketch with the curves is out of possibility because they contain parameters critical for the plane aerodynamics.

So, is there a way i can give solidworks a solid with an enclosed space in it, to obtain the solid for that space´s filling?

Thanks for your help.

SolidworksParts And Features