I thought starters are added by events created in the maker. The only script used is pbAddPokemon.
Now, I don't know how the player actually picks the starter in your game. If it's the "classic" way, like picking a Pokeball from the table, create a Pokeball for every starter, with 3 pages:
1) On the first page, make a message similar to "Would you like *insert starter name*?", and a Yes/No choice. If yes, then make a pbAddPokemon script, along with a selfswitch A, and a global switch that indicates that the player has chosen a starter already.
2) The condition for the second page should be the global switch - the graphic should display a Pokeball, and when the player interacts with it, a message should appear like "This is one of professor *professor name* remaining Pokemon", or something else that indicates that the player has already chosen a Pokemon.
3) The third page is blank, and the condition is the selfswitch. This way, the Pokeball the player chose will disappear.
That should do it, I think.