I don't think there is such a thing as a "focus keyword" like you describe. Search engines will take whatever information they please for ranking a site, not what you want them to use.
In Q2A you can add tags to a question, but those are just for categorizing a question within Q2A. And you can make the question title as the URL slug up to a configurable number of characters. You could also write a plugin that adds a keywords meta header to each page, but AFAIK that header is mostly ignored by search engines these days because it has been abused by spammers too much.
For general SEO you may want to take a look at the Ultimate SEO plugin.