I don't know if there is a code. But you can complete de Dex by seeing all Pokemon in platinum. All Sinnoh-dex Pokemon can be found ingame.?
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Diamond and Pearl Sinnoh Dex: 46# Wormadam Platinum Sinnoh Dex: 46# Wormadam
in sinnoh dex it is togetic but in national dex it is igglybuff
Swinub is #203 in the Sinnoh Dex in Pokémon Platinum.
In the sinnoh dex: porygon In the national dex: sunflora
go and see all the Pokemon in the sinnoh dex