Word doesn't have a feature that would join page spreads into single pages. Perhaps you are thinking about doing it in PDF format where these post-production features exist.

If you did your cover artwork in Powerpoint then you should have done the back/spine/front with cropmarks as a single graphic in that software.

Book cover artwork shouldn't be included in the same file as the internals of the book. The paper size it is printed on won't match the internal pages of the book and will not be printed on the same stock. If you want to include the cover artwork then just use the cropped front cover to match the other pages.

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Andrew Lockton
Chrysalis Design, Melbourne Australia