You wont be able to create a plugin to automatically fetch the metadata. But you can just create a new metadata reader plugin for your files that extracts the ISBN. That way you can just click a button and calibre will download all metadata based on the extracted plugin.
As for your question: Look in the calibre sources (in the builtins module) to see how metadata reader plugins are created. Create yours in exactly the same way and add it to calibre with a high priority so it overrides the built-in one.