It is indeed fairly easy to get speciatly dog foods in HK, but they can be pricy. If it is a food you can only get through a vet, try contacting a couple vets offices in HK to see if they can supply it beforehand. Royal Canin is definitely available in HK, but not sure about about the particular variety you are using.
Wise Pet -
index also imports various premium pet foods, but for specialty vet foods you will probably have to find a vet here.