David,
The best way to do this (if you are entering new items on the worksheet manually) is to make sure you are within the current boundary of the named range ("Servers" in your example), right-click, select Insert... then Move cells down in the dialog. This will extend your named range automatically. Enter the new item in blank cell and the validation list will contain the new item. This makes it easy to place a new item into an existing order. The only tricky situation is if you want to add the new item to the bottom of the list. Select the current last item, insert a new cell as before, move the last item up into the blank cell then add the new item.
Regards,
Mike