It is possible to date Katie before the San Fierro area is unlocked in fact, this can be done even before the first mission.
She likes to spend her spare time at the karate dojo in Garcia, San Fierro, San Andreas. She has had the job for three years, as she says to Carl during dates.
She is a nurse in an ER unit in an unnamed hospital. When Katie decides to do a driving date with Carl, awkwardly when starting (in front of her house), she will say that she doesn't like that place and sometimes says that she doesn't want to live there. Although she actually lives there, it is possible that she doesn't like her neighborhood. Katie lives in the Juniper Hollow district of San Fierro, at the north-west corner of the city, just south-east of the Gant Bridge and the nearest home to it.