This isn't possible with the stock Akismet plug-in.
While Akismet is very well integrated into WP, it's written to be the final arbiter of the comment. By that I mean it marks the comment as 'spam' after all of the other stock WP functions have run - including the whitelist.
If you look in akismet_auto_check_comment() you'll see that if the web service returns true, it installs a pre_comment_approved filter that simply returns 'spam'.
If you code, it should be very simple to either add a whitelist text control to Akismet and add a check of the comment against that or add a call to check_comment() (from /wp-includes/comment.php which will be loaded since it's the thing that's calling the plugin) and have that be a final savior of a comment from the spam bucket.
I like the check_comment() route because it approves tracebacks and pingbacks against your blogroll and approves comments from people with other approved comments.
If you want explicit control, the whitelist approach gives you that.
That I think hacking the Akismet plugin would be the cleanest way to do it vs writing another plug-in and trying to get it to work with Akismet.