There is a way to do this. Select the auction from post sale and hit the Relist button. From the relaunch page, hit Edit and on that page, check off the option that allows you to save the item to your inventory. Then save your changes. After this you can abandon the relist process as you no longer need it.
Now, go to your inventory. You will see your item there -- you can now edit it and fill out the Storefront Information tab to get it listed in your storefront as a fixed price item. I hope this helps!
