Can't they save the sprites of the new characters so he can introduce them to the game later?
you could start with honey the cat and do it in the poses that are repeated in each character and the poses that are not repeated then leave them undone so that they can then recreate those with new and different poses, but make them normal sprites from the original story and then modify the normals to make the sprites for that other mode.

wait... the robots of the stage 1 have motor oil and the characters can´t get pregnant with motor oil,the only enemy from there who have sperms is bokun, well... what is bokun exactly?, he is a robot too or not?, i can`t rembember him very well.