The only way to do this would be to trade to a copy of Fire Red, Leaf Green, Ruby, Sapphire, or Emerald, using a Gamboy Advance or Advance SP, and from there export it to Diamond or Pearl using the gameboy Advance slot on the ds..
U cant
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It can't be done directly. What you can do is transfer from XD to one of the GBA games (Ruby/Sapphire/Emerald/Fire Red/Leaf Green). You'll need a Gamecube (wont work with a Wii), a GBA (wont work with a DS), one of the games I just mentioned and a GBA to Gamcube transfer cable.
When you've traded a Pokemon to the GBA game, you can then upload it to Diamond through Pal Park (you'll need a DS for this)
You go to the city with the huge fountain in the center. To the left side of the fountain, the room with the PC in it lets u trade. talk to the lady at the front desk. I think you might have to beat the game first. Not Sure.Type your answer here...
first trade to a 3rd gen game or firered/leafgreen. then transfere to diamond.
Using the correct trading cable> :) i hope i was of some help
Thx!