I'm not familiar with any off-the-shelf solution for this purpose. Most commonly folks have the source code that generated the JavaDoc.
That being said, the overview-tree.html traditionally has a fairly straightforward HTML format.
It should not be difficult to write a script that would read the file as text or as a DOM, reconstruct the hierarchy of UL and LI tags, and use that to build an input file for graphviz. I've done similar stuff in the past with other forms of data.
It's just a matter of time and proficiency with the scripting language or appropriate tools.
The one problem of this approach is that you would only get the hierarchy of classes. You would have to make it somewhat smarter if you wanted to get the "implements XYZ" and create multiple hierarchies. Even if you could get that data, you would have to manipulate GraphViz's levels to get it to provide an appropriate layout once you have this multiple inheritance structure.
Of course, adding the details of the members would turn this into a whole new problem since you will have to access other HTML files.