Adoption

Looking to adopt my Thai girlfriend great niece to become sole custody and change surname for immigration requirements
Thanks Andy Ault

Get married and go through normal adoption procedures.  There are reasons for rules and procedures when it comes to kids. :/

This is for my girlfriend  only due to it is less paper work for her to adopt first and then we marry later trust me we have looked at all options we have the father and mothers signature on the correct documents but come to a dead end because we are not married

If you have already looked at all the options and have found yourself at a dead end, then I don't know what you are asking for on this forum.  Clearly your "less paperwork" route is not working for you.