There are several 'MakeFaces' tools to add faces to imported CAD line-sets.
If the CAD file is poorly executed you need to heal tiny gaps where ends don't quite meet etc - again there are several tools [e.g. TT's EdgeTools...]
Also if the lines aren't 'flat' they need to be 'flattened' - again there're several tools to do this...
You need to assemble a set of tools to tidy gaps, flaten, remove unwanted vertices, and add faces etc... They are out there in the Plugins Index...