sure, you would probably want to use isset so that a default is used if the session is not set;
if(!session_id()) session_start();
$kfm->defaultSetting('max_image_upload_width', ((isset($_SESSION['width']))?$_SESSION['width']:1024));
$kfm->defaultSetting('max_image_upload_height', ((isset($_SESSION['height']))?$_SESSION['height']:768));