There is a hook you can use to insert custom data before the actual creation of the event:

$post_data = apply_filters('stec_ics_import_post_data', $post_data, $item, $this->calendar_data, $this->initiator, $this->post_type);

E.g.

add_filter('stec_ics_import_post_data', 'your_custom_function', 10, 5);
/**
 * @param array $post_data - the stec_event post data array
 * @param array $item - the ics item object
 * @param array $calendar_data - the calendar data array
 * @param int  $initiator - the ics import initiator user id
 * @param string $post_type - the post type (stec_event, )
 * 
 */
function your_custom_function($post_data, $item, $calendar_data, $initiator, $post_type) {
    // ... modify $post_data here
    $post_data['stec_cat'] = array(1, 2, 3); // add categories 1,2 and 3
    return $post_data;
}