If your requirement is just to show the vertical tab stretched along the left side of RadDock - luckily, this is something that can be achieved. You need to handle the DockStateChanged event to set the desired size of the tab item there. In addition, you need to handle the PropertyChanged event of the strip that holds the item in order to change the size of the item(s) when the size of RadDock (and its left strip respectively) is changed. I am attaching a sample project that demonstrates the approach.
However, if you require something else, the overall behavior of the Outlook feature compared to the behavior of the AutoHide window is quite different and there is no straightforward way to achieve it.
I hope that the suggested tip has helped you.
the Telerik team