This isn't supported by the built-in export routine. One thing you could
try is replace the checkboxes in the report with textboxes formatted in
a Wingdings or similar font, containing expressions that give a tick or
a cross for -1 and 0 respectively (or Y and N or anything else).
Or take a look at the Report Utilities at http://www.lebans.com, which
give a much more faithful representation of the appearance of a report
when exported to Word.