iwork.xml

<?xml version="1.0" encoding="UTF-8"?>
<mime-info xmlns="http://www.freedesktop.org/standards/shared-mime-info">
  <mime-type type="application/vnd.apple.pages">
    <comment>Pages Document</comment>
    <generic-icon name="x-office-document"/>
    <glob pattern="*.pages"/>
  </mime-type>
  <mime-type type="application/vnd.apple.keynote">
    <comment>Keynote Presentation</comment>
    <generic-icon name="x-office-presentation"/>
    <glob pattern="*.keynote"/>
  </mime-type>
  <mime-type type="application/vnd.apple.numbers">
    <comment>Numbers Spreadsheet</comment>
    <generic-icon name="x-office-spreadsheet"/>
    <glob pattern="*.numbers"/>
  </mime-type>
</mime-info>

Drop this in /usr/share/mime/packages, then run sudo update-mime-database /usr/share/mime, and then you'll be able to use the file manager to set a proper association for these files (e.g. LibreOffice)