Making php work on html file - could work with powerCMS?

Hi,

Could I have your opinion on this?

I will probably have to build a CMS site for one of the site I manage. The problem is, the main page contents were made in htm format, but shopping sites are in php format.
I know the fastest way is just re-save htm files into php files, but rather not to reset SEO contents.

So I just found out there's a way to make php work on htm/html file using .htaccess file, which goes something like;

AddType application/x-httpd-php .php .html .htm

I'm wondering if this trick could cheat PowerCMS (as php code functions on html/htm files with this .htaccess control) , so I don't have to change the file format to php, or it is crucial that file extension is .php?