Can I use knitting wool for needle felting?

My answer is yes, you can needle felt with yarn. For the best results I would recommend to use yarn with high wool content, at least 50%. I personally find that it felts quicker and the bond with the other fibres will form better. Loosely spun yarn or rowing style yarn always works best, but it should not stop you to experiment. Also look out for wool which is not machine washable, as mashing washable yarns are made to prevent felting.

Yarns I recommend for needle felting  /wys-re-treat-chunky.html , /isager-aran-tweed.html , /rowan-felted-tweed.html