attaching a roof to the stuctural model

Is this possible? Structural has provided me with a roof structure to attach to, but I can't seem to select it. I have structural's drawing linked as a background (not sure if this is the best method for that sort of thing), and I can not select the roof joists as supports. I also can not copy/monitor them into my model. I searched all I could to find an answer, but the best idea I found was to copy and paste them from the structural model, and that seems like a bad idea for some reason.