So far as I'm aware you can't, you'd need to download the relevant data and crunch it manually.
You can, however, structure in a rollup account to generate reports for the future:-
No need to panic. This post aims to offer an approach to help you plan
your Google Analytics accounts setup in a structured fashion to help
with clarity. I hope that by following the approach and the technical
steps, you will be able to collect and manage all your data, make more
sense of it, and most importantly, ensure what you are reporting on,
trending, dashboarding and analyzing is based on accurate data.
Some further reading is available in google groups