I was wondering if I buy the digital deluxe version of the game now do I have to start a new account or can I simply add the code to my exising account and get the 30 days sub and in-game items?

If you want to add the in-game items to your account, then just enter the Key (login in first... then go) here... http://www.startrekonline.com/activate

(However, I don't believe the 30 days sub is now supported for existing accounts, only for new accounts... Cryptic changed their policy when the cost of the package dropped below the cost of the monthly sub...)

You just add it to your current account. All the bonuses will then be added to that account. The 30 days will be added to your sub time, like all keys added to an account do (except for unique keys, only one each of those).