We've often gotten this question from clients looking to squeeze every bit of marketing from their website, which is a wise decision; however, sometimes a "Contact" page is just a "Contact" page.
If you find that you cannot write (naturally) 400+ words on the page, then you will not have enough content in the eyes of Google to rank well for ANY KEYWORDS, no matter how easy they may be.
See CONTENT is KING and QUALITY is QUEEN. If you are looking to turn an "About Us" or "Services" page into a ranking page, you may want to re-address what keywords and the naturalness of it fitting into your page's description is.
A keyword does not have to be in the page title or URL to do well; however, this is one of the top ranking factors for Google. Therefore, we find it best to create brand new inner page OR blog post that is centered around your "1 keyword:1 page" rule.