The only way it would be possible is through a game genie/pro action replay, and I'm pretty sure even they don't have codes for it.

I'd find it extremely amazing that one person has managed to do this where thousands upon thousands of others have failed. Not to mention that there's no space in the party line up for anymore characters. With Crono, Marle, Frog, Lucca, Robo, Ayla, and (SPOILER)Magus, there is no room for anyone else. These supposedly secret characters are not shown in any ending, there are no weapons for sale for them anywhere, no rocks for them to triple tech with, and both are (SPOILER)dead/missing in all available timelines.. If you could play as Schala, wouldnt that (SPOILER)end Magus' search for his sister? and why would you be able to play as Cyrus when (SPOILER)one of the quests is to put his soul to rest?.

Wow, I ranted. Go me!