you cant
if its a regular Pokemon the original answerer is right but if it's a ditto and you are facing a legendary you can transform into it for the length of the battle or until ditto/legendary Pokemon faints if it faints during the battle.
Unfortunately, Rotom does not transform in pokemon diamond, just in platinum, heartgold and soulsilver
the legendary Pokemon in diamond and pearl are dialga and palkia.
when you are battling a Pokemon use transform and it will bvecome that Pokemon and know its moves and ability for this battle (Works On Legendary Pokemon Aswell)
No, a Bidoof cannot evolve into any Legendary Pokémon in Diamond.
Heatran
a lot of legendary Pokemon
Dialga
you cant unfourtanately
arceus
you can't catch it because you already beaten the legendary
you can't unless you have AR
no