How can I create a custom variable to use it in my layout template page?
Your approach is right but this has less to do with omega than with the order in which Drupal is preprocessing your hook. The html template hook is too late in the process, this is just for rendering the html wrapper and already assumes the existing variables. Instead, following the 1st handbook link above, use the base preprocess hook to establish your variables, ex:

function theme_preprocess(&amp;$variables) {
  $variables['my_variable'] = 'My New Variable';
}