There is no import sheet within Power Store that I know of. Perhaps a technical specialist can chime in here. The sheet I showed you was one I created from the database. I did my upload through the admin area of the database. That shows you which fields are available to be populated. You don't have to fill them all out but you can't go back later and add another field, that I know of.
I totally agree with you on the uploading feature, this should be included in the product. The store I ended up using has that and after trying to get 5500 products into Power Store, let me tell you I would never do a large store without that feature. It's still a lot of work making sure your csv files are properly set up but with proper uploading built into the product yuou can go back and change or append easily.
So as far as I know you are on your own. You have to create a csv according to the fields in the spread sheet I attached earlier or the database itself. Make sure you don't miss any important fields or you have to manually add them later. (If there is another way I would love for Web Assist to fill me in.) You can also create a csv for your categories if you like. The problem I experienced was when I tried to upload a csv to link all the products to their respective fields. It took me a day to get it right and the darn thing wouldn't work anyway. So you may end up having to do this all by hand anyway but if there is another solution I would love to hear about it.
Let me know how it turns out.