yes, copy your adjusted formation into one of the two free slots, then click "save formation" and save it to your HD or MU.

After that, exit and go into EXHIBITION GAME then select any two teams (it's not important!)
Go to the bottom option and select "load formation"... simply load your newly saved formation into one of the slots then proceed with the game. You can quit as soon as it starts then go into XBOX live.

Your new formation will automatically be shown in one of the two slots when you come to the selection and formation part before the game!