Hi Jean-Paul - currently MoI does not have any mechanism in it to do 2D filled drawings, so you do need to use a 2D drawing program such as Illustrator to generate that part.

The easiest way might be to use "Live trace" in Illustrator which can do a pretty good job of generating closed outlines around a bitmap just like the one you posted here - you may want to set some lighting options in MoI to eliminate the shading and just have pure white for the surfaces, see here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=3135.3

Then make a screenshot of that and bring that into illustrator and use Live trace, that will probably involve somewhat less illustrator-specific knowledge or lower level curve manipulation required.

- Michael