Custom Actions for an install

Does anyone know how I can get an installer to copy a file from a CD to the
installation path as a seperate action?

I have an installer msi that is responsible for installing the main
application. What I want to be able to do is deposit (at the root of the cd)
a file that contains encrypted expiry date information. This will be
alongside the msi on the cd.

When the msi has finished I want it to move the file from the cd to the
installation directory of the main application (as per path chosen by user
from dialog).

Because this date needs to be changed often I do not want the file to be
included as part of the installer msi. If it was I would need to rebuild the
msi with every change to the expiry date file. I just need a simple
mechanism for 'dropping' in a new expiry file on the cd and the msi (which is
always configured to move the expiry file) would know it is there and would
move it to the installation folder.

Any help appreciated