I believe this will do it!
(for general knowledge)
Now, for what you want to do,
You can write the nick to the file set a timer to delete it after X seconds.
And when the adding command is used you will check if the nick is in the txt file or if the timer exists. But i suggest checking the txt filenn because in case your mirc stops for some reason.. (like a failed while loop) the timers will stop.
P.S if the length of the list is not too big you can more easily add the nicks to a variable with $addtok and with a timer $remtok it!