I thought it would be nice to have transformation added, and possibly linked to word-choice because sometimes I know a word and want to get it into another form. I don't know the word in the other form. I think that's commonly known as transforming a word.
The answer to "Where do I ask about adding a tag?" is here on this very site; that is the answer whatever you mean where to ask somebody else to add the tag for you (which could also be done commenting the question that would need a tag that doesn't exist), or where to ask about the appropriateness of a new tag you think to use for a question.
I don't see much questions about transformation of words, to start using that tag. As simchona says, I fear that those questions would be closed as general reference.