Is there a 3-way occupancy switch with one sensor being external?
There are more choices than you might think

http://www.leviton.com/OA_HTML/SectionDisplay.jsp?section=38557&amp;minisite=10251

I am thinking you might want a 3-way, manual on, auto off.

But whatever Pattern, Coverage, Time delay, or Switch type, you will likely find it here.

You can get leviton stuff anywhere, it is likely one of these would be a special order, or buy it online.

Do not worry about bad amazon reviews on these things, they are usually a case of the switch being more clever than the installer.