AFAIK there are no tools or options within DokuWiki that will do that.
Which leaves you with three options that I'm aware of:-
1. Export all your Xwiki as HTML and convert that
HTML to DokuWiki syntax. Works well. I can recommend
pandoc*¹ from experience.
2. Export all your Xwiki to a Word Processing format and convert that to DokuWiki syntax. Didn't work well for me.
3. Create a script based on the
XWiki To DokuWiki Convertor Plugin and use that. Untested.
I have no tips for converting structure.
*¹ >V1.3 is required for DokuWiki output. The Debian Stretch package is V1.7 and installs into Jessie.