I am wondering if it is possible to add the HHonors number to the booking after it has been booked, and then receive the Diamond benefits (room upgrades, breakfast, etc)?

And is there any risk (or less chance) of these benefits not being applied if the number is added after booking?

The hotel we are looking at is the Hilton Imperial Dubrovnik.

Also, does booking 'The Great Getaway' sale have any impact on Diamond benefits?

Any help would be most appreciated!