I think your talking about Arceus, a special event Pokemon.
No, but you can transfer/trade Pokemon from Diamond to White.
Migrate from Pokemon Sapphire to Diamond then Diamond to Black/White
No, you cannot since Pokemon White has newer Pokemon
Yes, but only after you beat Pokemon White.
you can't get the pokemon off of pokemon black on diamond, but you can do it the other way around.
I am pretty sure.
You can not catch any of them in black or white, but I think that you can catch all 3 in diamond.
Transfer it from Diamond.
To transfer your Pokemon from diamond to white you first have to beat the game in Pokemon white. After that you have to go to the Pokemon transfer lab, go up the steps and speak to the scientist. Then you need another DS that has your Diamond Pokemon in it, go to DS download play on the main screen and you can choose six Pokemon to transfer from your storage. 1) You must have already beat the Pokemon White Game 2) You need 2 DS's 3)The Pokemon that you want to send to Pokemon White must be in your storage in the Pokemon center in Diamond.
You have to wait until you beat Pokemon Black/White and obtain the National Dex before you are able to transfer Pokemon from Diamond and Pearl.
Dawn never was on pokemon black and white she was only on pokemon diamond and pearl.
I'm not that sure if you can trade, but you can transfer.