I don't know that you can do that. You might want to look into making a hidden div which you can toggle the visibility attribute on.
here's code that does what i'm talking about, except you would have to work on making your div look like the dialog box you want.
HTML Code:
<html>
<head>
<title>Toggle Div</title>
<style>
#mydiv {
background-color: #CCCCCC;
border: 1px solid #000000;
width: 200px;
height: 100px;
visibility: hidden;
position: absolute;
left: 150px;
top: 50px;
}
</style>
<script>
function toggleDiv() {
var o = document.getElementById("mydiv");
o.style.visibility = o.style.visibility=="visible" ? "hidden" : "visible";
return false;
}
</script>
</head>
<body>
<div id="mydiv"> </div>
<a href="#" onclick="return toggleDiv();">Toggle Div</a>
</body>
</html>
i could be wrong though. you may want to search on the regular javascript dialog box thing, but i've never seen one.