Lufthansa recruited onto the A300 fleet right out of flight school and still recruits directly onto the MD11 if any external recruits are taken. Just need 1000 hours minimum.
Anyway, 777 and 787 are on a common typerating and mixed flying of both types is perfectly possible and requires a lot less training than training new guys, but thats a decision any airline has to make wether they want to use that possibility or not.