It's still throwing the "File is read-only" error on my face every time I try to edit a code with a Story Editor. However, I have found a work-a-round that works in my case. Create the file manually:

In location:
\Divinity - Original Sin\Data\Mods\<YourMod>\Story\RawFiles\Goals\

Add a new text file with a name you want and with this content:
Quote
Version 1
SubGoalCombiner SGC_AND
INITSECTION

KBSECTION

EXITSECTION

ENDEXITSECTION


You can then move it under "__Start" section in the Story Editor and begin editing it.