Hi S,
yes, you can add the consignment information to a listing in your Saved Listings prior to posting it using the consignment check-box beneath the description editor. If you refer to listings on your Closed Listings page that have been imported from eBay, you can use the Import Listings button on the Closed Listings page to import those listings into your Saved Listings so you can work on them. You would be able to add your consignment information from there.
If you have more specific questions or encounter any trouble, please feel free to file a support case from the Help tab of your account and we'll be happy to help you out.
- Craig