Stemming isn’t really intended to handle transformations like "America" to "American" (or vice versa), because they don’t share a common morphological root. Stemming generally aims to reduce inflected forms of a word to a base form (e.g., "running" to "run"), focusing on suffix-stripping rather than transforming between nouns and adjectives.

I created a script and achieved the same results with Porter Stemming as you find. However, using Lancaster stemming from NLTK I achieved Runner --> Run