Just set the variable 'page_cache_maximum_age' using variable_set in custom module to the value 31536000
variable_set('page_cache_maximum_age', 31536000);
If you install the Devel Module there is a page for viewing and editing variables.
Above is a quick fix. If you want to do a perfect way then you need to form alter the "system_performance" form and change the option of below field.
$period = drupal_map_assoc(array(0, 60, 180, 300, 600, 900, 1800, 2700, 3600, 10800, 21600, 32400, 43200, 86400), 'format_interval');
$period[0] = '<' . t('none') . '>';
$form['caching']['cache_lifetime'] = array(
'#type' => 'select',
'#title' => t('Minimum cache lifetime'),
'#default_value' => variable_get('cache_lifetime', 0),
'#options' => $period,
'#description' => t('Cached pages will not be re-created until at least this much time has elapsed.')
);
$form['caching']['page_cache_maximum_age'] = array(
'#type' => 'select',
'#title' => t('Expiration of cached pages'),
'#default_value' => variable_get('page_cache_maximum_age', 0),
'#options' => $period,
'#description' => t('The maximum time an external cache can use an old version of a page.')
);