Well if by sock you mean a mesh bag big enough to hold 10 lbs of grain then you sure can. That's what BIAB is. You won't be steeping it though, you will be mashing. You have to maintain temp to make sure you extract the max amount of sugars from the grain.
That being said, I had a ton of fun building my mash tun from a cooler, and I barely get a 2 degree drop over a 60 minute mash. It's really all preference, but if you plan on doing 5 gallon batches, I recommend building a mash tun.