1) Right click on the task bar.
2) Scroll down to "Add application to panel"
3) In sub-menu, select "Add non-KDE Application"
4) In the Button title field type: "Power Off" as your label.
5) In the Description field type: "Power off system"
6) For the Executable field, click on the folder icon to the right of the field and navigate to /sbin/poweroff and select "ok" (if you have a hard disk or USB installation, you may need to place the sudo command in front of /sbin/poweroff, e.g.:"sudo /sbin/poweroff").
7) Check the box "Run in terminal window" (this is needed for sudo users, who will need to enter their password in the terminal screen).

To change the icon theme of your button:

1) Click on icon box in the top left of the configuration screen.
2) Use the drop down menu and SCROLL UP to get to the "Actions" set of icons.
3) Choose between one of the two, red, poweroff/exit styled button icons (there should be a round one and a square one) and click ok. I chose the square one, to differentiate between the system default log off button and this custom one.