As pointed by user55548 here you have an approach for this How to make payment method visible in admin order creation
However, you should be aware that using...
protected $_canUseInternal = true;
Will show payment method as available when creating new orders through admin (which could be a problem if there are some customer service users working in the project)
You should play then with app/design/adminhtml/default/default/template/sales/order/create/billing/method/form.phtml template (or maybe rewrite Mage_Payment_Block_Form_Container::getMethods() to add your custom logic) to exclude that method when creating admin orders