I'd rather this was simpler, but you can do it like this: (presume you're in a 'tell [task]' block for convenience):
copy note's last character to end of note
set note's last character's style to missing value
set note's last character to "\nNew line of text here"
Of course, if there's no note to begin with, this won't work. However, in that case you can just set the note to whatever you want.