"There isn't an obvious way to add sites to Safari 4 Beta's new Top Sites page, but this is a Mac after all, so it occurred to me that drag and drop might work, and it does:
Open the top sites window.
Click the Edit button.
Open the web site you'd like to add to Top Sites in another window.
Drag the want-to-add site's site icon (it appears to the left of the URL) from the URL address bar into the Top Sites window. You can position the site while you're dragging it, too.
Drop the site when it's in the position where you'd like it to stay -- the site will be pinned to this position automatically."
macfixit