You *can*, but it has next to no insulation value, and can become a mold farm. Mainly all it does is decrease drafts coming between boards, not that that's not worthwhile but there are usually better ways to do it and it does not perform the same function as actual insulation.
If you are looking for inexpensive or free sources of insulation, I recommend getting in the habit of asking stores if they have any sheets/blocks of styrofoam, recieved as packing for merchandise, that they are wantin' to get rid of. (Cover it with [scrounged] plywood or panelling, and make your carpentry tight and gap-free to keep mice from getting in there)
If you are looking for ways to gainfully employ used feedbags, there are a bunch of things you can do with them -- the paper ones are great under mulch to suppress weeds in the garden or to smother out new areas for future gardens (takes a year or so to work fully); the plasticky ones can be made into tote bags e.g. for shopping or the beach, or a variety of other less-fancy purposes.
Good luck, have fun,
Pat