Things you can do here:
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Yes and No.
Yes. If the pages are BRAND NEW, without forcing the URL to appear very spammy, we would recommend using some form of the keyword in the URL. It can't hurt, unless you really SPAM the URL and make it look unnatural.
No. If your pages are old, indexed, have page rank, have age, have value, don't change them. You'd be shooting yourself in the foot. A big part of the SEO game is age. By changing the URL, Google will believe the new pages you created are NEW . . . because they are and you will lose most of the value you build from the other pages, even if you setup a redirect.
Hope that helps.