I was wondering the same today, and after some search, it seems that it is not feasible.

Despite the AWS UI gives the feeling that the secret key/value pairs can be added or removed independently, the secret value is a JSON object.

Consequently it is only possible to read or write the object as a whole, and not a subpart of it. Which makes sense anyways, when thinking that the encryption/decryption mechanism applies to the whole content of the secret, and not to each key/value pair.