Make a gedcom file on Ancestry of the family you want to export, import the same file to Geni.
Please, be careful not to make doublettes that need to be merged with profiles of the same person already existing on Geni, because it will be a huge and timeconsuming job merging.
You could make gedcoms of smaller parts of the ancestry tree, importing them to Geni in smaller parts of your tree on Ancestry and wait a day or two to let the system look for matches.
