Sebastian Leupold wrote
The bottom is obviously implemented using 3 panes side by side.
Yep, after looking at it again, it's arranged in 3 panes... It was late when I looked at it last night and only saw the ContentPane div, didn't notice the 'ThirdPane' divs setup across the bottom... what can I say, my ability to divine layout from rendered source isn't as great late at night 
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So, to answer the OP's question, yes either a custom skin or you could also attempt to modify the skin you're currently using to include more panes. How you go about this will depend alot on how the skin was created (pure CSS Div/spans or Tables).
A simple explanation is that a pane is created by insterting either:
< div id="MyPaneName" class="MyPaneClassName" runat="server" > < /div >
or you can also make a pane out of a TD element:
< td id="MyPaneName" class="MyPaneClassName" runat="server" > < /td >
Notes:
1 - The code above has extra spaces before and after the < and > you probably want to remove those when you go to make your own, I put them in so the text editor doesn't eat it thinking it's real HTML
2 - One thing to keep in mind is that every skin file must contain at least one pane ID'd as "ContentPane" without it, you will get errors when you load the page.
3 - I find editing the ASCX files to be the easiest way, but if you're just starting with skinning DNN you probably want to use HTML/XML skin pacakges if possible. However, if you're working with a purchased / licensed skin, they may not provide HTML files, and you may be forced to edit the ASCX. Don't panic, it's very similar. Just make sure to backup the working skin.ascx file before you start.
4 - Consult the documentation for more details on skinning DNN sites, there are a lot of handy bits of info in there.
5 - You may want to checkout the Wrox books on DNN, or even "DotNetNuke for Dummies" which has a good explanation of the HTML skinning process for DNN (don't know if it's been updated for v5.x yet though).
Good Luck