I know it can be done but I don't know HOW. How do I add endorsements to a puppy already registered but still in my name?

Best to give the KC a ring, I should think.

It has to be still in your physical possession. 

All you need to do is write to the Kennel club with the particulars of the dog, same as when removing them.

Cheers -dog still here so no problem.

The exact details for what you need to do are on the KC website.

"Alternatively if the dog has already been registered, the owner will need to write in to the Kennel Club requesting that an endorsement be placed upon the records of a dog. The letter should clearly state the breed, Kennel Club registered name and Kennel Club registration number for the dog, and that the owner has physical possession of the dog. The owner should also enclose the registration certificate for the dog so that this can annotated accordingly. A letter of confirmation will then be sent advising that this request has been carried out".
http://www.thekennelclub.org.uk/item/365