If you need only fixed amount discount, then you can remove the validate for Discount Amount field so that you can add negative value in this field, so when you try to apply this coupon it will automatically add that amount instead to decrease. So you need to override below two classes.
For more details on Magento override see this Link.
Mage_Adminhtml_Block_Promo_Quote_Edit_Tab_Actions
find this code
$fieldset->addField('discount_amount', 'text', array(
'name' => 'discount_amount',
'required' => true,
'class' => 'validate-not-negative-number',
'label' => Mage::helper('salesrule')->__('Discount Amount'),
));
and change it to
$fieldset->addField('discount_amount', 'text', array(
'name' => 'discount_amount',
'required' => true,
'label' => Mage::helper('salesrule')->__('Discount Amount'),
));
and remove the below code
if ($this->hasDiscountAmount()) {
if ((int)$this->getDiscountAmount() < 0) {
Mage::throwException(Mage::helper('rule')->__('Invalid discount amount.'));
}
}
from
Mage_Rule_Model_Abstract::_beforeSave()