Primecoin use a different type of work, they compute prime numbers (instead of computing SHA256^2 hashes like regular bitcoin).

To use them, p2pool would in addition a plug-in that enables it to compute the primes (just like adding support for litecoin to p2pool required adding a library to enable it to compute Scrypt hashes).

As far as I've searched, there aren't currently any such plugin written for P2Pool (and myself, I lack the python skills needed to write it, I only program in C/C++).

But don't despair, as XPM rises in popularity, someone with the correct skill set will eventually step-in and write the necessary plugin.