I highly doubt that is possible. The two formats you mentioned are conceptually not compatible with XSL:FO. Word|Writer Documents are for formatting text and embedding pictures, XSL:FO is for defining where on a page data should go...
Anyway, Writer Documents are XML as well. You can write a XSLT to convert the XSL:FO to a sxw. But there is no guarantee that it will appear like it did as an XSL:FO document.
You can always print to PostScript and convert to PDF.