Hi movieguru,
Thanks for posting your question here. If you would like to create categories within your eBay store, you will have to do that directly through your eBay store editing interface once you have subscribed.
Once you have created your desired eBay store categories, you can then import those into your Auctiva account by mousing over the "My Account" tab within your Auctiva account, selecting the "eBay" option under "Settings", and then clicking the "Update Store Categories from eBay" button on the ensuing page.
Once you have done so, you should see two new drop down menus on the lister page within your Auctiva account called "First eBay Store Category" and "Second eBay Store Category" from which you can specify up eBay store categories you would like your item to appear in during the listing process.
It is not possible to set your eBay store categories before creating an eBay store subscription, so I think the categories you referred to that were already set in your Auctiva account may have been the primary eBay listing categories you had selected when creating your items.
I hope this helps. If you need any further assistance with your account, please feel free to contact our customer support team by mousing over the "Help" tab and selecting the "File Support Case" option.
-Mike