Absolutely!!

It depends when the product came out though. I couldn't add the license for Quake 4 since Q4 came out a year before Steam added ID stuff to their list of purchasable games. But apparently if its a product that is available on Steam, even if you buy it on the shelf, you can just install it via Steam and type in your serial num.I purchased UT3 via steam and it has all of my info including serial on my steam account.