This is a hack, but assuming that your document won't be damaged by undergoing some slight changes, here is what I would do.
- Logon to www.iCloud.com, and open Numbers. If it isn't there, go to beta.iCloud.com.
- Upload the new Numbers document to iCloud and then open the document in your browser.
- Select Download a Copy from the settings menu (as seen in the screenshot below) and choose to save it as a Word document.
- Open the Word document in Numbers and save/export in whatever format you wish.
You could, depending on the nature of the document, send it to a friend who has Yosemite and ask them to export it to the Numbers '09 format.
- If I were you, and did want to mess with importing/export I'd probably try CloudConvert.com.
- This article claims you may be able to revert the document, although the fact that you can't open it in the first place would seem to negate that option.