How to convert surfaces to solids in Rhino3d?

select the poly-surfaces. This have to be intersecting surfaces and/or polysurfaces. They will be trim into a closed polysurfce.

then command: CreateSolid
or see menu solid=> Create Solid

BONJOUR SOLID=POLYSURFACE PARCEQUE SI VOUS DECOMPOSER UN SOLID VOUS AURAI PLUSURE SURFACE EST SI VOUS JOINIER LES SURFACES VOUS AURAI UN SOLID OU UNE POLI SURFACE ESSAI DE CRIER UN SOLID ACTIVER FILTRE QUI SE TROUVE EN BAS A DROITE EST SI VOUS SELECTIONER POLISURF LE SOLID EST SELECTIONER

Without seeing what kind of surface you are speaking about it is hard to answer. There are several on line utilities that will turn a nurbs surface into a solid .STL. Some surfaces will simply need to be capped..... no naked edges or bad geometry.