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a) add the keyword phrases to meta tags of the existing site
b) add the keywords into the body of the page
c) add a new page to the site and focus on the keywords there
d) deal with the expense of buying and adding a new domain, specific to one keyword phrase and focus just on it
I am second guessing if simply adding the keyword phrase to an existing site once or twice would be enough to lock in that search result. Another consideration is that adding keyword phrases involves rewriting an existing page thereby changing its flow. Same would go for metatags. Any suggestions would be great as I seem to just be rambling. Thanks.
Because you said the new keywords are related to a site you already have i would recommend (c), too.
But besides this i would consider what jrobbio said about (d). Maybe it's a good idea to buy a new domain if you expect your competitors will otherwise do this.
Organising your site concerning keywords try "theme pyramids":
Hope that helps.