I had a similar question. I found a thread on this forum discussing how to move substitutions from one version of OSX to the next. I adapted this solution to moving system substitutions into pages. I wrote a shell script, which could be run with the cron job above.
The script relies mostly on PlistBuddy - a preferences editor supplied with the OS by Apple. You will need to edit it before running so that directories in the script match those on your machine. Other than that, my script is well documented, so I won't discuss further...
# copies text substitution preferences from
# system to pages
# Author: Craig Champlin
# Adapted from: http://apple.stackexchange.com/questions/57960/how-can-i-export-text-substitutions-from-lion-for-import-into-mountain-lion
# not robust at all, quick and dirty - use at own risk
# change to preference directory to save typing
# backup preferences for pages
cp com.apple.iWork.Pages.plist com.apple.iWork.Pages.plist.bak
# extract system substitutions to a file
/usr/libexec/PlistBuddy -x -c "Print NSUserReplacementItems" .GlobalPreferences.plist > ./zzzSystemReplacements
# wipe out substitutions in Pages preferences
/usr/libexec/PlistBuddy -c "Delete kSFWPAutoSubstitutionArrayProperty" com.apple.iWork.Pages.plist
# create a new, blank substitutions entry for pages
/usr/libexec/PlistBuddy -c "Add kSFWPAutoSubstitutionArrayProperty array" com.apple.iWork.Pages.plist
# merge exported preferences into pages
/usr/libexec/PlistBuddy -c "Merge zzzSystemReplacements kSFWPAutoSubstitutionArrayProperty" com.apple.iWork.Pages.plist
# visually insure operation completed successfully
/usr/libexec/PlistBuddy -x -c "Print kSFWPAutoSubstitutionArrayProperty" com.apple.iWork.Pages.plist
As a caveat about using text substitutions in Pages, Pages does not convert the font style like TextEdit does. This means that special characters appear in whatever font you happen to be using. Since most of my substitutions are math symbols and since my default font is Helvetica, my substituted characters usually appear as a dot. I have to change the font style for these characters to Cambria for them to appear correctly.
If you didn't know this, you might assume that the substitution had not worked.
Best of luck to ya!