You can not you have to transport it from emerald.
How To Catch Deoxys In Diamond Without Action Replay?
You can you have to go to the guy in palpark hes at the top floor talk to him hes beside the books he will say there been a disterbence at mountain cornet check it out and there will be a pokemon deoxs and you can capture him you have to have you national dex tho hes lv 70 wen you capture him