Create nested set model without top level item

I'm creating a nested set model for a taxonomy database. Basically, I've re-created the same type of setup that wordpress uses for taxonomy, however instead of using parent_id for the term_taxonomy, I'd like to implement a nested set model for taxonomies that are hierarchical.

The issue I'm finding is that terms aren't placed into the term_taxonomy table, until there is an actual term or category name. However, with a nested set model, you need one entry to be that top level item.

Is it possible to do a nested set model, without having a top level? Instead you could have 5 top levels and subsets, et开发者_开发技巧c.