You cannot add extra content placeholders since the system only knows about leftContent, mainContent, and rightContent.
Also these content place holders are expected to be inside divLeft, divCenter, and divRight Panels respectively and you should not move them out of those. You can place divLeft, divCenter, and divRight anywhere you like in the markup including above wrapcenter (btw wrapcenter is not required).
However, note that even if you move the divLeft or divRight to a different physical location in the markup. You should look in your stylelayout.css because if these items are using postion:absolute; then they will be positioned accordingly no matter what their physical location. Several of the included skins do use absolute positioning for left and right but not all of them do. So you may need to remove the absolute postioning from the css to achieve your desired layout.
Hope it helps,