there are many things you can do with your wool sweater- start with washing it- hot water- detergent, cold rinse- hot dryer- when it comes out you will have a wonderful piece of felted wool to be used any way you would like- as batting, as wool applique- as penny rugs...recycled wools are wonderful to work with!
if you decide to use it as batting you will want to (try) to match weights/lofts with the other fibers you use- i recycle alot of batting but i keep poly's together, cottons together, wools together---but as long as they are about the same weight/loft-and can be cared for the same way it might be ok to mix them. wools are so great to work with after you felt your wool you may decide on something different to do with them=
a couple places to check out for using recycled wools are:
Sue Spargo
Primative Gatherings
the Cotton Patch Quilt Shoppe- in East Tawas, Mi---they have a link on their web site to their Wool site
Wool Central
there are many more- those are the ones i can think of right off hand...but start with washing/drying your wool-then you can cut it up and do what ever you want with it.