The stone you want to add will work well with your combo stone. The Naniwa superstone 5K is a good stone. It's soft, wears a little quickly and polishes really well. The superstones really shine (pardon the punn) in the upper grits and I don't personally care for them in the lower grits.
I do think there are some good competing options in that general grit range and price range.
Nubatama Bamboo 5k is a winner. Ken did a good video on the stone on the page so you can watch how it works and get a little lesson on sharpening at the same time:http://www.chefknivestogo.com/nuume5kst.html
The Suehiro Rika is a good stone. I've sharpened my way through a whole stone and it's just a pleasurable stone that works well on a wide variety of steels. At about $50 it's a good deal. I would choose this one over the Superstone:http://www.chefknivestogo.com/suri50grst.html
There are a few other good options but I don't want to overwhelm you.