BuddyPress.org

Use this filter hook and write a function:
apply_filters( 'bp_members_format_activity_action_new_member', $action, $activity );
Found in : ...plugins\buddypress\bp-members\bp-members-activity.php

For example, untested:

function pixie_new_member_activity( $action, $activity ) {

    $user_id = $activity->user_id;

    // assuming the field names are State and City
    $state = xprofile_get_field_data(  'State',  $user_id,  'comma' ); 
    $city = xprofile_get_field_data(  'City',  $user_id,  'comma' ); 

    $userlink = bp_core_get_userlink( $user_id );
    $action   = sprintf( __( '%s from %s in %s became a registered member', 'buddypress' ), $userlink, $city, $state );

    return $action;
}
add_filter( 'bp_members_format_activity_action_new_member', 'pixie_new_member_activity', 10, 2 );