What if you just made a single slap menu item and then had the output be random? If you want to slap with something specific you could always just type it out, as I'm sure finding the right item on the list is equally (if not more) difficult than simply typing the message. You could even shorten what you type by making a slap alias in your aliases file. Something like "/slap /describe $active slaps $2 around a bit with a $$3-" would do the trick.

If you happen to like the idea of a random slap, I'd look up $read() and consider storing the possible slaps within a plain text file in your mirc folder.