It is not possible. Spore is a Ciderized game. That means EA wrapped the windows binary so that it can be ran on Macs. There is no way to get that code running on PPC systems.
I agree, it is a shame that they decided to go Intel only for Spore (and several other games). There are plenty of PowerPC G5 and several G4 based machines out there that would be capable of running Spore if it would be coded for PowerPC.