Any chance for alter system settings form via hook_form_alter? I would like to create module based on other custom module. One thing that will be nice to able to do is to alter module settings form fields. Is this possible?
Do not forget at the end of your hook_form_alter() or hook_form_FORM_ID_alter():

$form = system_settings_form($form);


If you don't want to add the "Save configuration" button, you have to add your custom callback submit with $form['#submit'][] = '_my_callback_form_submit'; and : 

function _my_callback_form_submit($form, &amp;$form_state) {
  variable_set('my_variable_name', $form_state['values']['my_variable_name']);
}