Complex Surfacing

Hello all! I'm a former plastics engineer with some experience designing using surfacing methods. I used Siemens NX to do this at work. My current side project involves a bit of surfacing as well, but I think I might have hit the limit of what Onshape can do...The project I'm referencing is a public document, just search "Project Zephyr" and I think it will come up.

So I have cross-sections that I'm currently lofting together to make 1 surface, in order to get all the varying shape and control that I want. At the front I have an orthogonal cross-section, so it comes to a point. Both surfaces should be coincident, and tangent (G1 curvature, but I think G0 would work, I might like G2, but not sure) at the boundary. Aft end is currently open.

http://imgur.com/a/1Ysjk

So I'd like to get this to a point where I can import it over to Simscale, and do some CFD work on it. I think Simscale will need a solid model imported.

My question is: Can I merge these surfaces, or close off this volume with other surfaces, and fill up the volume to create a solid part? In NX there was a "sew" command to join surfaces, and a "trim bodies" command to cut away material from solid blocks. I can't seem to use multiple surfaces with the split command, nor can I sew the surfaces together. Any other options?