Please see my answer to your other identical question.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
Once you are married your husband will automatically become your children's "legal stepfather." I think the question you may really be asking is how can your fiance legally adopt your children and become their legal "father." If that is what you are really asking this can be done through a stepparent adoption. Your ex-husband, the father of your children, will have to agree, unless there are extenuating circumstances. There are self-help books explaining how to do a stepparent adoption. Nolo has a very easy to follow book which does just that.