This isn't currently possible. Feel free to log an improvement request for this feature in our public facing JIRA instance (https://jira.atlassian.com) and file it against project "FishEye".
Perhaps developing a plugin is an alternative path forward for you:
Hi Community! I’m Elaine, Confluence Product Manager. You may have read my earlier post about page tree in space navigation sidebar. I'm excited to share another improvement that helps you organize ...
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