Ok there might be a hard way to do with using a messenger script...
But it is quite easy to do it with autohotkey (www.autohotkey.com)
Just make a script with the following code
code:
#ifwinactive ahk_class IMWindowClass
Enter::Send, {Shiftdown} {RETURN} {Shiftup}
#ifwinactive ahk_class IMWindowClass
+Enter::Send, {RETURN}
this would require an extra program to run constantly on your computer so in that way it isn't as nice as a plus script but if I understand Cookie and Markee right, to make such a script is way hard so I'm afraid there is a good chance it will never be made...
[edit]
This exe should do the trick...
As Ezra suggested you could make a script that executes/terminates the exe with Messenger starting up/shuting down...
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