Agree with martinibuster, If you are serving your visitors really helpful information by adding more content to your website, it is not going to harm your ranking. Just you need to organize the content that you plan to put on your website. If you are going to add a blog to your website it could be more helpful for your visitors to get some extra knowledge or information about the posted product or service in the blog (if you posted one). You can also get suggestions through the comment facility by your visitors. It could be helpful in improving your product / service quality. You can also hyperlink your important key phrases that you want your visitors to follow or visit.
A good interlinking and continuous link development of internal pages could get you a nice ranking in search engines with your most hyper linked words or phrases.