your kind of missing the point.
you are trying to select a Power CMS Content Area and convert it to a Power CMS Content area, that will not work.
the two tools where not intended to do what you are asking about, thats the bottom line. I understand that is not the answer you want to hear, but that does not change the fact that it is the answer.
it is not intended to migrate your site from Power CMS to Power CMS Builder.
the Power CMS of Power CMS Builder is no different than the Power CMS of the Power CMS Solution pack. you don't "Upgrade" your content areas by using the Power CMS Builder Dreamweaver interfaces.
The only way migrating from Power CMS Solution to Power CMS Builder in your site will work, is if the power CMS Solution pack was installed at the root of your Dreamweaver site. if this is not the casein your site, than the migration is not possible.
if it is the case, there is nothing special that needs to be done. You can select an area of HTML Content and use the PowerCMS Add Content to PowerCMS Option to convert that HTML Area to a new content area, or you can use the Insert Content From Power CMS Option to insert a existing content area to a page in Dreamweaver.