Can I sew in non-feltable embellishments after knitting is finished, but prior to felting?
You can try it, maybe on a small swatch but why not wait to see the finished size of the felted item before sewing on embellishments?
I don’t know but I think it won’t work out well since the idea of felting to shrink the item and your embellishment probably won’t shrink the same. That’s the reason for preshrinking materials used in sewing projects; some will shrink, some won’t, or they might shrink at different rates and everything is ruined. As salmonmac says you can try it. If it works, I’d like to know.