If she can't be adopted as such, which I don't think she can as an adult, she can change her surname and you can all have a 'Linda's new name' party ...or something similar.
Thanks for your answers I did read about 18 but wasn’t sure if there was a “loop hole”
She already has her step fathers surname when we married I changed it. We were going to apply for adoption (when she was 9) but didn’t want to make contact with her biological father to open a can of worms that didn’t need opening, now wished we did as it’s too late