Not possible as one always needs to press the RETURN key manually if a chat message should be send to others.
This means you'd need to press RETURN each time for every user on your friendslist.
Addons could only post the message to the chat text edit, that's all.
So, in addition to Pchat, or light stand alone addon, ability to make broadcast to all in friendlist! Or/and, ability to make lists to make broadcasts. So there would be lists called /friends1 /friends2... and on settings of that addon you could define certain people into those lists @account1 @account2....
So to simplify :
When write on ESO chat:
"Hello" text goes to all in friends1 group, and friends1 group contains all accounts you configured in it in settings.