You can transfer funds back and forth between your Sportsbook (Main) account and Casino account as often as you like. There is no charge for this, however Sportsbook transfers are not eligible to receive Casino bonuses (only Poker bonuses).
To make a transfer from the Sportsbook into the Casino, select "Poker/Casino Transfers" under the My Account menu, and then "Transfer From Main".
To send the funds back, return to the same page and use the "Transfer To Main" option. Your funds will be immediately available.