You could probably use a sewing needle for this. Thread the feather stem through the eye of the needle and hten pierce the worm body down the middle until the end pokes through. Then pull the feather end out of the eye and remove the needle. Never done it, but I don't see why it wouldn't work.
There's also a similar technique with thread you could probably do as a pull through. Same approach you use with tying off thread wraps in rod building.
Use the quill of the feather to push through the body... Snip it off when you hit the size you want, put a small drop of crazy glue on the quill and pull it back into the worm... Been doing that for years..