Since Xmap 1.2 you can adjust the page priorities for any page in your sitemap without affecting any other page. To do that just follow the following steps:

1. Go to the home page of your site and login as a Super Administrator user
   http://www.example.com   (not the administrator page)
2. Then visit your XML sitemap:
   http://www.example.com/index.php?option=com_xmap&view=xml&sitemap=1
3. Look for the page you want to modify and the click over the current page priority or change frequency of that page. A drop-down menu will be diplayed with the available options for that setting. Once you have chosen one optiion, the sitemap will be saved automatically.