Just about to start the same boat and I have the same question. Just tagging along.  

���I was hoping to get an answer to this too. Any ideas?

 I have no experience with the Mill Creek but since no one else responded I took a look at the construction gallery pictures. It appears that the seats sit / attach to simple wood rails attached to the hull.  I would take a seat, possibly velcro it into place and try paddling solo.  Once you have a location figured out that balances things, follow the same instructions as for the first 2 sets of rails and add a 3rd set amidships.

Seems like that would do the job.

Ed 

I recently finished a Mill Creek 16.5 and I can't see why couldn't add extra seat position. Are you building the wooden slat seats as per the plans? If so then you just need an extra set of seat cleats in a mid-ship position and corresponding foot braces. You'll also need an extra set of the buckles that hold the seat back in position, but that's no problem. I'm not sure where the mid position would exactly be, but another option could be to make the front seat cleats longer to provide more seat position options. I made my cleats adjustable with multiple holes, so that I have 6 inches or so of adjustment in their position. I did this to allow for pasengers with different leg lengths. The front one I made longer than the seat so that I can attach a removable tackle box behind the front seat. I have some pictures on my blog of the cleats, but it doesn't show the adjustment available.

http://millcreekkayakbuild.blogspot.com.au/2015/02/seat-cleats.html

   Thanks.  I bought a creature comfort seat and placed the back 2 feet from the aft bulkhead as someone had recommended.  With the velcro, it was a pretty low risk proposition.  It worked great yesterday and seemed to be a good compromise forward and back.  The only issue I found was that the kayak is widest at that point and my paddle seemed too short.  I kept banging either the shaft or my arm on the sides of the kayak.

 For mid ship paddling you may want to get a longer paddle.  Allows lower angle to get paddle to the water. 

Alternatively raise the seat an inch or two.  Will give slightly higher center of gravity and slightly less stability but in a mill creak that shoud not be that much of an issue.

Ed