Sorry, but you can't Rowan. I've tried it myself and it didn't work. Hope this helps!
You have to battle them.
Just go to Pokemon platinum central and click on pokedex and it shows you where to battle all the Pokemon in sinnoh.
If you see a Pokemon in the battle frontier you have not yet seen, it will not record it. The only way you can add that Pokemon to your pokedex is if you see it outside of battle frontier.
you get the sinnoh pokedex from professor Rowan afteyou battle the starly with one of the Pokemon in the bag and say yes to the professor and you get the Pokemon and the pokedex.
yes you see them but it does not go on your pokedex sorry
YES.
no they don't add Pokemon into your pokedex there
this question is worded weird but if your question is "where do you get all thePokemon in Pokemon platinum?" then the answer is to battle every trainer in Sinnoh.
first you have to battle with every pokemon like i did! it took me a long time but i did it on platinum i also have all legendary pokemon
fill your pokedex and battle more trainers
u need the national pokedex!!!!!!!!!!
You pretty much fill the pokedex, battle, trade, and chat with your friends, and play in the underground.