It is not currently possible to associate listings that are already active on eBay with the consignors you have created within your Auctiva account, however, we will most likely be adding such an option with a future update of the Auctiva website. With our current system, the only way to associate an eBay listing with a consignor is to check the "Is this a Consignment Item?" box on the Auctiva lister page during the posting process.
Any additional questions or concerns? Please feel free to contact our customer support team using the appropriate link on our help page: http://www.auctiva.com/free-auction-software-help/help.aspx