Embedding an existing blog in iWeb is easy using iFrames
open an html snippet, and insert the code:
PHP Code:
<iframe src="your target url goes here"
style="width:900px; height:1200px;
border-width:0px;
border-color:#990033;
border-style:solid;"
scrolling="auto" >
</iframe>
Make sure to insert your target url, and alter the width and height to suit