Please take a look at my blog. I believe you want something like that.
If this is in fact what you want, then do this:
1. Write your welcome message in a text widget and place the widget at the very bottom of your sidebar 1. The text widget should have this:
[Your welcome message here]
2. Now, in your CSS, add this code:
border:solid 1px #00f;
For your customization, I'm assuming you have the "Add to existing CSS (View stylesheet)" radio button checked.
You may need to play width the height and top properties of the .welcome class selector and the top property of the #primary and #secondary id selectors. Make sure the top property in the .welcome class selector is the negative equivalent to the value of the top property of the #primay and #secondary id selectors.