One option would be to build a page with content from the pages you want to include using the Include Page Macro (https://confluence.atlassian.com/doc/include-page-macro-139514.html) or the Excerpt Include Macro (https://confluence.atlassian.com/doc/excerpt-include-macro-148067.html)
Then you can export the page with your desired content.
If you don't want to create a new page for the export, you will still need to bring all the pages to the same space in some way.
When they are in the same space, you can use Content Exporter for Confluence addon to export a whole page tree or any selected children of a page:
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