Okay, so you might have clicked on this post to disprove me, however I have found enough evidence to suggest that it would be possible to create a mod that adds full multiplayer functionality to spore, albeit it would be very difficult.
Just recently we have been able to create mods for spore that involve actual coding, the limitations aren't documented, but theoretically we could create a system for players to see eachother ingame.
2: Save syncronizationhttp://forum.spore.com/jforum/posts/list/86820.page
This right here is a post showing a turn based multiplayer system, however that's not what I want to show you, it's what one of the players is experimenting with, currently somebody called DOCC_KYLE is working with a program that would allow for 2 players to play at the same time in the same galaxy VIA save syncronization, go to page 26 of that forum to see what he means.
The concept of mixing the save syncronization DOCC_KYLE is fiddling with and a Planet=Server system (Basically every time a player goes onto a planet they host a P2P server for others to join by entering the same planet, if another user enters the same planet, they would be put into the server with player 1, player 2's ship would be rendered on player 1's screen using either something simular to a disguised gate from GA (Not the same thing), or an NPC ship.
Also, just to let you guys know, DOCC_KYLE is still messing with the save sync system, however even if nobody makes a mod like this, we still have the good old turn based multiplayer that we currently have.