PowerCMS is meant to be used to control the content of pages, or parts of the pages. If you are looking to give your client full control over the content on the pages then this would be the rite tool to use.
You would need to change any pages that you would like to control with PowerCMS to be .php type. I'm not sure about doing the rewrites through the htaccess but if you just updates the links in the site to be php page it should be indexed the same, you would probably need to submit a new site map also though.
I'm not an expert when it comes to making changes and the effects they could have on SEO so you may want to check into this part of things a little more before you deploy PowerCMS.