My husband and I recently found out that we could add a fathers name onto my child's birth certificate even long after the birth of my child. If my husband is not the biological father of my child is it leagal to add his name on to her birth certificate and if so would we then have to change her last name? Or will she keep the same last name?

No of course it is not legal to do fraud and put down a name that is not the real biological father,

The real father who ever he is, is the father, not your husband, if your husband wants to adopt the child, you can go to court to have your husband to adopt.

Totally agreeing with chuck... you are committing FRAUD if you knowingly put someone else's name on your child's birth certificate when you know they aren't the father.