Obj cube in zbrush from rhino question


I exported a obj file from rhino, then imported it to zbrush.
The object has a cube underneath it. The problem I got is that I use brush and alpha to edit on the top organic part, but the cube surface I can’t. It only allows me to move the four corners of the cube. It seems the cube only has four polysurface to it???

What can I do to make both the organic part and the bottom open to texture edit?