Either paternity is established or it isn't. If it is, there's no means by which you can fraudulently represent your fiancé as the child's father, as the child already has a legal father. If it's not, it's still not a good idea to engage in fraud.

If you simply want to petition to change the child's surname, you can do so once you're married. If dad is long gone, no known forwarding address, you can get the court to authorize substituted service.

If your fiancé wants parental rights, with a full understanding of what that means, the proper means of achieving that end is through stepparent adoption, once you're married.