How to thicken a face not a surface ?

Hello,

I am currently in the following situation: a designer gave me a surfacic file exported from Rhino in STP format.

I imported it into Solidworks and it created in an imported surface in the tree.

Now, I need to thicken this surface to create a shell so the part can be plastic injected.

I have already tried to create a solid and to shell but it doesn't work as the surface got some radius smaller than the thickness of the shell needed (3mm).

So I was thinking that I could thicken faces on a one by one basis and to arrange the radius issue locally.

Yet , I found out that thicken can be applied only on surface ?? not on face.

How to do then ?

Regards

SolidworksParts And Features