Code:
<PanelFile ID="ID_UISkin_Toolbar">
<Element ID="ToolbarField" X="0" Y="508" Width="1024" Height="200">
<Element ID="Toolbar_WebstoreButton" X="1" Y="1" Width="63" Height="66"></Element>
</Element>
</PanelFile>
That example would move the store button to the upper left corner of the mainbar
Note that the moment you modify anything in PanelFile ID="ID_UISkin_Toolbar" your skin will lose the ability to adapt in different resolutions. The above example would always set the mainbar on the left side of the screen no matter what resolution you use ("ToolbarField" X="0"). That would mean if you want your users to have the mainbar centered they would need to edit those values themselves or you would have to provide the values/skindefs yourself.