sitemap.xml and sitemap.php are already there. Do we have to add new added pages in sitemap.xml every time we make new pages?
yes, redo it, and then resubmit it to google...they have a resubmit function in the webmasters tools...
Or perhaps do it once a week. You can probably find a way to automate website scan and sitemap creation.
If you have a lot of spare time, you can do it. But I will suggest you spending the time on most important SEO issues, like submitting to directories, social bookmarking sites, etc.
I think we need add new page in the xml file provided you know xml which is really simple to learn and code.