I wanted to kick of DIY Week with this tutorial because I had so many requests for it :) If you don't know what or how to use expandable posts, click here for my first tutorial on Expandable Posts.
So, lets get right into it....
Check your "Expand Widget Templates"
and find (command F) this line of code:
Now select the above code, all the way down to </b:if> (shown below, it may vary slightly)
<a expr:href='data:post.url + "#more"'><p align='left'><img border="0" src="Your Button URL Here"/></p></a>
You can change where your button is aligned by replacing "left" with "center" or "right"
And Voila! You have it!