扩展组成的Go Struct的标签

Question

I have two structs:

// package a -- third party library
type UserProps struct {
  FirstName `json:"first_name"`
  LastName `json:"last_name"`
}

// package b -- my own package
type User struct {
  a.UserProps
}

The problem is that UserProps is inherited from a 3rd party library, so I can't change its source code.

My question is can I extend the FirstName tag from within the User struct in package b so that the tag would eventually be as such json:"first_name",sql:"unique_index" ?

Also, note that I only want to extend the tag of only one field, FirstName.