With the Heat?
The only way to do it is to turn on trade override. You'll have to sign Dalembert or Chuck Hayes.
Heat Trade: Bosh, Dalembert/Hayes, Two First Round Picks
Magic Trade: Howard.
Then do another trade.
Heat Trade: Mike Miller, Shane Battier
Magic Trade: Turkoglu.
So the Magic will gain Bosh, a starting C (Dalembert/Hayes), Shane Battier, Mike Miller, and two 1st round picks.
I did two trades because you can only trade 4 players/picks from one team at a time, and in order for the Heat to get Howard they would have to give up more than that.
While this makes the Heat VERY thin, such things are necessary when trying to trade for BY FAR the best center in the league.
EDIT: Ok, so this year you can't sign a FA and then immediately trade him, so you'll have to wait 30 days in order to execute this trade.