After looking into the code, it seems there isn't any user-exposed functionality for this, but you can do it quite easily if you don't mind editing the source code. There's an array PHRASING_ELEMS in Readability.js (line 159 at the time of writing) that determines which element tag names to preserve.

You'll need to modify that array yourself to achieve what you want, either by editing your local copy of Readability or by making a fork of the source code with your custom array and then using that in place of the original (or even better, fork and change it so that users can define their own included/excluded element arrays in the constructor options to solve the root problem here).