Hi DRUN1111,
While you can't import from a spreadsheet, fortunately there are ways to speed up the process of entering things manually.
In terms of searching for cards when you don't know the year/set/specific insert, I highly recommend searching the player and using the filters. The Advance Search (button next to the Search button at the very top) can help as well.
When you know which cards you have, on the set page after clicking Collection Summary, you can simply use "Add / Update" to add your cards to your collection, plus there's options for your Wantlist and Trade List as well. In this view, if you click the down arrow button between the checkbox and card number, you'll see an option called "Add From Here..." that lets you add a whole sequence of consecutive cards. If you don't want to have to do that multiple times for larger sets, I highly recommend going to your Profile-->Settings-->Site and changing "Rows to show on checklist page?" to 1000.
You'll see you can also "Add by Card Number" which is useful if you have a list. The two "Add Multiple" options are excellent when you have differing numbers of multiple copies of cards.
I'm sure others who add larger amounts of items way more often than I do can offer even better tips, but for now I can at least mention the above and say that Admin has continually added new functionality that makes it less laborious to get your cards added. What used to take days can now often be done in a matter of hours.