There are numerous occasions where Sinnoh pokemon appeared in the Advanced generation, an example would be that two of the Advanced Gen movies were based on sinnoh Pokemon.("Lucario and the Mystery of Mew" & "Pokémon Ranger and the Temple of the Sea" where Manaphy was shown) Brock also had captured a Bonsly durning the Second half of the Advanced Generation. Chatot may have also appeared towards the end of the Battle Frontier season if i remember correctly... Hope this Helps :D