Example of adding systems to stages nested in deep schedules, with bevy 0.5. Trait to add feature to AppBuilder - add_system_to_stage only works if the stage is in the root schedule.

bevy_nested_schedule_stage_adding.rs

pub trait AddSpawnSystem {
    fn add_spawning_system(&mut self, system: impl Into<SystemDescriptor>) -> &mut AppBuilder;
}

impl AddSpawnSystem for AppBuilder {
    fn add_spawning_system(&mut self, system: impl Into<SystemDescriptor>) -> &mut AppBuilder {
        self.app.schedule.stage(stage::MAIN_SCHEDULE, |main_schedule: &mut Schedule| {
            main_schedule.stage(stage::SIMULATION_SCHEDULE, |sim_schedule: &mut Schedule| {
                sim_schedule.add_system_to_stage(Stages::SpawningStage, system)
            })
        });
        self
    }
}