When you enter a product into PS3 it goes immediately into the database. The data is stored there, there is no flat file that contains the data. So yes you can enter it manually, then export it as a csv, or you can create a csv and import it. You can give ti a shot entering a few products and then going into your database to get the columns correct. The one problem you might have is that some changes are not importable directly. For example You can import long and short descriptions but let's say for example you want a bigger font type on the long description. This is not an importable field even it you want it across the board. This or any other text attributes have to be set manually. Using firebug to check, just to be sure I find that the text is in a <div id=ProductLongDesc> There is no css style to edit this, it is an id so it has to be edited for each product. (If you haven't used firebug consider it. I just found it and don't know how I've lived this long without it.) :-)
If you already have the products in a spread sheet then you just have to add the correct column titles, as per the database, or the sheet I attached, save as a csv then import into the database directly. As of yet PS3 does not have a feature to handle this task. I think the evolution of PS will be in that direction. When you get into hundred plus categories and or thousands of products it becomes essential.
If you would like to speak to me directly I'd be happy to share what little I've learned. firstname.lastname@example.org