Extending default MODx Database Tables

Let’s say I want to build a snippet or custom manager page that wants to augment the default Revo database tables. Now, I know I don’t really want to mess with the original tables... I’m thinking more of making my own table with a 1 to 1 relationship with one of the existing table’s keys.

However, how is this best done? Normally, wouldn’t you edit the XML definition for both tables to include references to each other? But I don’t want to mess with the original table’s XML definition for the same reason I don’t want to play with the original table itself, know what I mean?

The table I want to extend is the categories table -- I don’t think that makes any difference from a conceptual point of view, but it may be useful to understand what I’m trying to do: I want to key my table off of the category ID, and then add a few additional columns to my own table, e.g. a integer column that specifies a custom sorting order.

Any recommendations on how this is best approached?