Based on some research I did, You can do this in the Terminal with this command:
hdiutil convert /folder/folder/file.iso -format UDRW -o /folder/folder/file.dmg
Make sure to replace /folder/folder/file.iso with the full filepath of the .iso file. To ensure correct filepath, simply drag the iso file into the terminal window at the point where you'd normally type the filepath.
NOTE: I haven't tested this yet. Make a copy of your .iso file before you use this command, just in case.