Can you use somebody else's `/efs` folder? So, assume that there are two Galaxy S3 phones (i9300)s. One of em has a "No Service", "Baseband Unknown" problem while the other works like a charm. Both of them are rooted and flashed with custom ROMs.  (CM11 and CM12).

So, if the phone with proper /efs makes a nandroid backup of /efs and then that backup is restored in the phone having issues, then this should technically work.  

Question is, will it?
No, this won't work, because there is cant be two phones with same IMEI codes and every single phone probably has special modem codes so efs from one phone won't work in other phone