ZBrushCentral

Smooth Group imports and bump generating

Hi everyone !
http://s2.noelshack.com/old/up/rugerwip9-6ab6776626.jpg
So , I’ve made a low poly mesh in 3ds max with smooth groups , I’ve unwrapped it and import it in Zbrush (thanks to the plugin which allows to import with smooth groups). After , I’ve sculpted it and I wanted to make a normal map generating test to know if it worked good.
Very bad surprise, even with the cage, all the mesh is generated (the cage is supposed to generate only where I have sculpted something).
I’m sure it comes from the smooth groups because i’ve tried with the same mesh without importing the smooth groups and it works well with the cage.
Someone knows a solution to my problem ?

EDIT : http://s2.noelshack.com/old/up/rugerexplanation-1ee2ed7a73.jpg a drawing to understand better what I meant