Yes, you can add your affix to your dog (as long as the registration has been transferred to you) at the end of your dog's name, whether or not is has a breeder's affix at the front.
The breeder is the only one who can have endorsements lifted, except in rare occasions. Have you asked the breeder? Endorsements are usually placed by a breeder in order to make sure that inferior stock is not bred from. Most breeders will lift the endorsement quite happily if your dog is 'up to standard' and has had the relevant test done for that particular breed. The Kennel Club will send you a new certificate with the endorsements lifted.
Even if the endorsements are not lifted you can still have an affix added to the pedigree.
Affix goes at the end of the name, UNLESS you bred both parents - i.e. if I bought in a pup and bred neither or only one parent, she would be 'xyz abc at Brihow'.
However, if I bred BOTH her parents, the affix would be added at the front, and the pup would become 'Brihow xyz abc'.
Just to add to the debate - "Affix" goes before the name and can only be put their by the breeder. If you are granted an affix of your own and want to apply it to one of your animals that you did not breed it becomes a "Suffix" as Brihow said eg: Blah Black Sheep at/of whatever. Good luck
To be strictly accurate: Your KC Affix can be used either as a Prefix (before the name - when you bred the puppy or both its parents) or as a suffix (after the name - when you didn't breed the puppy)