You can't. The association  page only starts when you
are "associating" a workflow  with a list.

An initiation page, however, will allow you to gather input from
users when the workflow is started....ie when items are created. It
sounds like you need to use an initiation page for your workflow.
You should be able to find plenty of information out there on how to
do this.

So, the association  setting can only be set once. Rather restrictive.

The association  page gets set whenever you attach a workflow  to a
list. You can remove it and add it back which would allow you to
change the input, but yes this is really intended for input that will
remain the same throughout the life of the workflow.

I see it as though you are really given the best of both worlds. If
you need to provide less dynamic input to your workflow and don't want
to bug users every time a workflow starts, go with the association
page. If you require greater flexibility than this and users don't
mind providing input every time the workflow starts, you can use the
initiation page. It is really a design decision and each one has its
advantages depending on what you need.