Convert imported file to solid - pb in grasshopper with BREP

Hi,

I'm importing an assembly part from inventor to Rhino.

To do it, I export assembly.iam into  assembly.stp file.

In Rhino, i select the block that i decompose in order to seperate parts.

I run Grasshopper to animate parts, i create BREP. But parts are not considerated as Solid. I can't select them.

How can i convert a block into solid in rhino ?