Within the USAF, you must be a Dan rank to wear a Hakama.
Unless you are female. It follows decency that a woman shouldn't be undressed in front of men. ( gi's are underwear.) Therefore any girl 5th kyu or higher will be allowed to wear a hakama.
However, individual dojo's have jurisdiction over hakama use according to the instructor's preferences. Practice can be different from dojo to dojo within the USAF. Some USAF dojo's don't allow anyone to wear hakama until dan ranking. Other's allow ranked women and dan ranks. Some allow men to start wearing them at 1st, 2nd or 3rd kyu(especially if they are instructing a class.) It depends on the head instructor.
But, typically I'd say it isn't a good idea to visit another USAF dojo, or go to a USAF seminar unless you adhere to the hakama regulations they have, despite what your dojo does. It wouldn't be good to be a 5th kyu man, and have everyone assume you are at least a shodan. There is some assumption that a woman could be any rank, thus a little chivalry is spent in introducing your self to chicks before you start throwing them.(at least in my experience)