Can we customize global configurations ( configuration.php ) to add custom parameters? . As it stores DB / FTP settings, I want to add payment gateway configurations. If there is a way to add configuration / parameters in a component then that will be helpful as well.
First: Use a current Joomla. Your 1.5 site will be hacked in no time.
Second - the answer (Joomla 2.x - 3.x):
You can add component params to the component configuration. (You can access the component config in the backend. Just navigate to the component and click the option button in the toolbar.)
How to add / extend component params:
Access the params in the fronted view:
$params = JComponentHelper::getParams('com_yourcomponent');
$test = $params->get('param_name');