Hi Leanna,
No, unfortunately, it is not currently possible to add a robot.txt file to your Auctiva store. The only customization options that are available for your Auctiva store are those displayed under the "Store" tab within your Auctiva account.
Any additional questions or concerns? Please file a support request using the appropriate link on our help page:
http://www.auctiva.com/free-auction-software-help/help.aspx-Mike