I want to add an offline payment such as PO method so that it appears just in the admin and not in the front end.
Is there a way to do that?
I guess you should create your own simple payment mehtod module.
Then you should check these properties for the payment model:
//You won't allow method to be used on checkout protected $_canUseCheckout = false; //You allow the module to be used on adminhtml protected $_canUseInternal = true;