Hi Doug,
Unfortunately singular-plural conversion is way more complicated than that. It requires very advanced linguistics, which is beyond the plugin capabilities (if considering multiple languages).
There is however a method, that may work for most words, when using the index table engine. The only way involves some custom coding:
1. First you need to back-up all your server files
2. Then adding this code to the functions.php file in your theme directory.
3. If you are not using the index table engine, then first you need to configure it. Then re-creating the index with the code active, should add detected singularized/pluralized keywords as well.