AFAIK there's no easy way to assign an action to a "single middle mouse button click on a nick". ("on hotlink"-events will come into conflict with the default mouse event settings).
But if "double-click the nick" (in nicklist or channel text) is OK, you could replace your current "double-click on nick" action (whatever it is) with the one requested.
To do this, go to options > irc > mouse > "command to perform on nick double-click:". In the box for "nicklist", you enter:
Then, add a new alias to your
aliases section (scripts editor, tab "aliases"):
nickclick { editbox -ap $editbox($active) $$1 }
...This will
add the nick you clicked to the end of the current line of the editbox of the channel.
If instead you want the current line to hold
only the nick (clearing all existing text in the editbox):
nickclick { editbox -ap $$1 }
And if you like, you can add control codes or extra chars arround the $$1 part (representing the nick), e.g.:
nickclick { editbox -ap 7[ $$1 7] }