Hello again:

Thanks Michael and Pilou. I had forgotten the thread you told about.
Actually, the Pilou´s method in Sketchup has been very very quickly and easy. (well, not easy for my computer, who spent almost an hour for draw the surface). The problem is that I can´t manage this file, because is so big and complex that it crashed always I´ve tried to export the file.

I have tried to export it with the Sketchup 7 pro EVAL -I have trying during the 8 hours for evaluate...what a pressure :-) - but nothing. Always crashed.

The other option is to start and finish this project in Sketchup, but I don´t feel comfortable with this software (it´s very easy but too new for me).

Well, I must to decide what are the best steps to attack this project.

P.S. :The option to extrude the curves can be the best. I didn´t want to make that because this obligate to me to close all the curves, but I have spent all the day probing options and surely in this hours all the curves could be closed and the volume finished :-)