I saw in the official FAQ (can't be arsed to dig out link) states that once created a DK has normal character transfer rules, which means that it can be transferred to a realm where no 55+ character exists on that account.

Edit: imo 10 boxing only adds value for pvp or 10 man raids, will not help at all with leveling etc - And adding 5 melee characters won't make life easy on you.