items that customs conficates get destroyed / thrown out.
Knitting needles will be no exception. In most countries anyways the customs officers would get in bad trouble if they did not follow that (stealing of valuable goods has happend - taking them instead of throwing them out).
They would keep stuff for reasons of evidence if they needed to. But knitting needles are not going to bring down a plane or anything, so probably not.
So: Trash can.
But some officers are nice and let you pack your things into an envelope that you can address to yourself. Then you pay shipping for your own goods, but you do not lose them.
It depends on the cost of item and cost of shipping if that is worth it.
If they want to confiscate, it is worth asking if you can mail it. If they do not let you, I don't think you should argue. That's never a good idea at customs.
I would recommend to take an envelope with you that is big enough and that you can address to yourself in case. maybe even carry stamps in the right amount so that you can put them on.
But really: you can also buy cheap needles and be better of (well, not if they let you work with them and you hiss and curse!)