Adding a new entry to the d20 npc wiki isn't
too complicated, but does require a little explanation.
Step one is to go to the "All monsters by CR" page here:
http://d20npcs.wikicities.com/wiki/D20_NPCs_by_Challenge_Rating
Scroll down to the CR area you want to add it to, and click the edit button. For the most part, you'll be clicking the edit button right next to the "Epic (CR 20 to 159) NPCs and Creatures" listing, or possibly the "Cosmic" listing.
If the CR you're adding already has a section, put the name of your creature under there, surrounded by [[ ]] marks, and followed by a <br>. If not, add your CR surrounded by === === marks. Basically, follow the format given by the other entries, you'll do fine. Save the changes.
Click the red link you just created. It'll take you to the page to add your new creature. Add it in. Then you're done!
If you're intimidated by wiki-markup (slightly different from HTML and board markup) you can go to this page:
http://d20npcs.wikicities.com/wiki/NPC_Templates
Click edit next to the format you want, copy the text there, and modify it as you need before dropping it into your creature page.
By the way, I statted up A'tuin like I said I would earlier, just for fun. I used the Dragon Turtle as a base, removed some of the dragon powers, then raised its size to Mega-Gargantuan. I think this is currently the largest statted creature in existance right now. It has nearly 11 billion hit points and more than 15 million damage reduction. It resists 5000 fire, cold, electricity, and 6000 of radiation, which should nicely enable it to fly through space. These stats should be similarly in line with the stats of a planet (a planet would be somewhat tougher, being made mostly of iron, but then, a planet can't move on its own and dodge/deflect stray comets) so consider this a LOWER limit for planet-busting spells and weapons.
http://d20npcs.wikicities.com/wiki/A'tuin_the_Star_Turtle