No, these cannot be combined. If you have a promotion code it can be used to save even more money when you are purchasing new certificates with a credit card or PayPal.
When you have a Restaurant.com Gift Card or eGift Card you may redeem it towards a Restaurant Certificate of the same face value or lesser value. For example, if you have a $50 Restaurant.com eGift Card, you can redeem it for a $50 Restaurant Certificate, two $25 certificates or five $10 certificates.
Since you are not purchasing a new certificate but redeeming one certificate for another, you are not able to use the promotion code.