I wouldn't suggest it, but my research leads me to believe it would likely work for awhile...
"The shell for inexpensive helmets is just stamped PET (the material used for bottled water containers) or a similar plastic. It is usually glued onto the liner, then taped around the edge for appearance, although some use no glue and others have no tape.
For more expensive helmets the shell is included in the mold when the liner is expanded from the bead, and must therefore be polycarbonate or another higher quality plastic that can take the heat of the mold (PET would melt). Really sophisticated techniques can add more than one shell section to the mold, and up to five pieces can provide shell protection for the lower section of a helmet, or even the interior. In this case no glue is necessary, since the shell is bonded to the liner in the mold. This technique could yield stronger helmets, but the designers use it to open up larger and larger vents and reduce the foam, eventually just meeting the current impact standard.
Skate-style helmets and a few bicycle-style helmets have hard shells made of ABS or polycarbonate plastic. BMX helmets (and a few older bicycle helmets) can have composite hard shells, with layers of fiberglass or even kevlar fiber laid up in an epoxy. ABS is molded, but Fiberglass shells are generally laid up by hand."
"The hardest and strongest whitewater kayak helmet shells are made from carbon fiber, Kevlar, and composite polymers. Plastic helmets are the lowest strength helmets although they are well proven, having been used for decades in whitewater."