You say you are "Not speaking of eBay's categories."
Are you talking about categories in your eBay Store? If so, you will have to create the categories you want through your eBay Store. Then you can place them in the desired category through Auctiva.
However, if you actually mean "eBay's categories" I believe you will be defeating any purpose you might have for listing on eBay.
First, I haven't checked it out, but I am sure you cannot create, or post, a listing without using an eBay category. I'm pretty sure it will get kicked back awaiting a category selection.
Besides, if you could, how would anyone ever find your listing. eBay categories are used for prospective customers to search and find your items.
I do not have, never have had, a designer created template (I am really satisfied with the many templates Auctiva has to offer), but you will still have to use a listing page to create your listing. This will either be on eBay, in TL, or Auctiva.
In the Listing page you will be given the opportunity to select your Custom template.
Sorry for such a long answer. I'll try to be less wordy next time.
