Thanks. As soon as anyone says "3-way switch" to me, my eyes glaze over. I
can never figure out anythig that has to do with 3-way switches, a red wire
in a circuit, etc. So, what you wrote is probably correct, but I am not
sure if I am brave enough to give it a try. I wasn't even thinking about
the fact that the motion sensor needs to always be powered in order to work
(duh), so I don't know what that will mean in terms of my idea.

David Nebenzahl wrote:

I know what you're trying to do. I think you can do it, but not quite
the way you had described. Here's what I'd do.

(I have wired outdoor lights using separate remote motion sensors,
which are easily available and cheap at places such as Home Despot,
so I have some experience with this setup.)

What a guy would want to do is to use a so-called "3-way" switch to
switch between the dawn-to-dusk and motion-sensor functions for the
light.
Since the motion detector requires power, you wire it so that it
always receives power to its black wire. You take the switched leg of
the motion sensor (red, I believe) and wire it to one contact on the 3-way
switch. The other contact of the 3-way switch just goes to power. The
common terminal of the 3-way switch goes to the light. (And, of
course, all neutrals [white] are wired together.)

This way, when the switch is in one position, it receives power from
the motion sensor *when motion has been sensed*. In the other
position, it always receives power, so it does its usual thing (come
on at dusk, go off at dawn).

Would require experimentation, of course, to see, for instance,
whether the motion-detected switching interferes with the
dawn-to-dusk logic. But in theory, it should work.