If you mean get the previous Legends onto Diamond, you must defeat the elite four and complete the Pokedex (just "see" the Pokemon). Then, you will be granted access to Pal Park, where you can import your Legends from previous games. You cannot return them to the original games so think carefully.
the legendary Pokemon in diamond and pearl are dialga and palkia.
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
no
you can't unless you have AR
Empoleon is not a legendary Pokemon. It is the final evolution of Piplup, one of the starter Pokemon in Pokemon Diamond, Pearl, and Platinum.
That would have to be Dialga.