You don't want to add anything about the easter egg map to the worlmap.txt. It's not a random encounter at all, but a forced encounter like Lexx said. It uses a global sfall script that looks for if you have finished the game and if you you are on the world map, it will then load map 166 (easter egg map).
So numerate your encounters normally and don't worry about the easter egg encounter.
OR wait... I see, you are talking about that line in worldmap.msg.
I'm pretty sure that is just a leftover from an attempt to make the end game encounter in a normal random encounter way.
I believe you can simply replace those "gathering" lines with your own encounter description. Or if you prefer to keep them, just send them down the list to after your own lines.