I don't know of a plugin but I have done this some years ago. It's a pain. Create a VB script to export all OneNote docs to .html, convert .html to DokuWiki format. The last step is the painful bit as you need to analyse all the styles in use, determine which features can be supported with DokuWiki and Plugins, then create the required conversion script with suitable regex.
If you've only got a small amount of entries to convert it's probably simpler to just copy and paste into a text editor (e.g. Kate/
Kwrite) that has builtin regex, convert the plain text copy to DokuWiki syntax and paste into new DokuWiki pages.
See also:-
perl HTML2Wiki,
DokuWiki tips:htmltowiki,
DokuWiki tips:doc to wiki syntax (skip the first step)
HTH