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With enough effort and the WIA SDK (and probably the Windows DDK as well) you probably can. But it will be a large amount of effort, especially compared to the price tag of a cheap scanner.
I'm assuming your time is worth something. If this is a hobby project, then compare the price of a cheap scanner to the time saved that can be spent working on the fun parts of the project. If this is a work project, then the time saved is more valuable to your customer than to you, but there should still be business case for buying hardware that will save more time than it cost.
Well, try if this virtual webcam supports the WIA interface: http://www.soundmorning.com/
If so, you are all set and ready to go.
You can also search for "Fake webcam", there are many versions.