So drag and drop the files onto the folder. Or select all the files and cut and then open the folder and paste them into it. Either way you should be able to add files to any read only folder. All my folders are read only and i have upwards of 30GB in some of them. No issues.
I do not accept support questions via EMail, PM, IM or my G+ page!
Phone: LG Optimus G Pro
Running: Stock JB from LG with Nova Launcher