Dont think so
After you see all the Sinnoh Region Pokemon, go to the Professor, and he will give you the National Pokedex, which includes Pokemon from the Kanto, Hoenn, and Johto Regions.
You can't catch or obtain other trainers Pokemon in emerald.
Trade.
there are 150 kinds of Pokemon that you can see and/or obtain. Unless you have friend trade you Pokemon that you can only get in pearl to you, you'll never obtain them all. In Pokemon diamond and pearl all you have to do is see the Pokemon and you get its data unlike the older versions where you have to obtain them.
Migrate it.
Well it depends if you just want to see them or HAVE them.Battle lots of trainers and walk through tall grass a lot if you just want to add them to your pokedex but you NEED to trade and all that to obtain every pokemon.
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darkrai and arcus
You can't. You could obtain the other trainer's Pokemon in only Pokemon colloseum or XD.
a REAL diamond? lol in a cave, if the Pokemon one, download it or buy it
The only way to obtain Articuno in Pokemon Diamond is to trade it from GBA via Pal Park.
No, you do not need diamond and pearl to obtain all the Pokemon in the national pokedex. You either need to have a friend who has all the older Pokemon, or you could transfer the Pokemon yourself from older games.