There are many ways you can do that. It would depend to some extent on the type of fly. All you are doing is to apply a means that you can wind a hackle around.
You can use the stem of the hackle for one, post wing yarn, foam, deer hair etc will all work.
Do you have a specific kind of fly that you intend to do this with.
There are hooks that also have the post fixed, as of course a metal addition.