I would build the content to support the additional phrases. If you target say "dancing shoe" and would like to attract traffic on "fancy dancing shoe", you need your pages to support this phrase too. A good rule-of-thumb is that you shouldn't put in keywords in your meta that is not found in your copy.
Do some research on related searches to your specific keyword phrases and extend your pages to support these phrases. Or simply build new pages to target more phrases.
Spamming the meta keywords won't do the trick for you.