If you're talking about the Kokuintou Hozuki, that is in the game and can be unsealed.
But, yea, adding new weapons to the game would require some pretty extensive modding, and would likely just crash multiplayer altogether.
I'm trying to play on my ps vita. every time I launch the game I get an error that says "the game could not be started". This happens with the original phantasy star portable as well. All the other psp games work just fine, and I've seen people...