You'll have to resize and move the mesh. I found a tutorial
at GoS - it's for clothing, but I think it uses the same principles.
You will need a mesh to convert a hair, and you can't in any way convert things with Bodyshop unless you've got a mesh to use for the textures. Sometimes you can use similar meshes for clothing texture conversions, but that's the exception.
You can do some conversions SimPE, but the only always successful conversions are between same gender Adult/YA/Elder, plus M2F/F2M kid and toddler. Sometimes it works between the genders for teen and adult too, like accessories held in a hand or a big costume. Adult meshes can't successfully be converted for teens in any cases with SimPE unless you make a new mesh.