Use the magic wand tool. Select the wall tool in the toolbox (one click) and hover the cursor over a line while holding the space bar. Click mouse. A wall will be created to the line (and any other joined at the end). In practise the DWG will have too many lines and will be joined at "inappropriate" places such as windows.
Cheers John
John Hyland : ARINA : www.arina.biz
User ver 4 to 12 - Jumped to v22 - so many options and settings!!!
OSX 10.15.6 [Catalina] : Archicad 22 : 15" MacBook Pro 2019 [/size]