1)I wouldn't do it the way your suggesting, for one thing you would need to put the Fostex in an enclosure, because the woofer would modulate the Fostex causeing distortion and peaks and dips in the mid's response.
2)With added enclosure to the Fostex, the volume would be less than 2.5 cu.ft meaning you would need to re-tune the cabniet with a different port tube.My suggestion would be make a cabinet the same size as the base of this (width and depth) calculate heighth of cabinet and put the 10" in there, that way you have a choice if you wanted to make it sealed or ported sub with out having to mess with an enclosure behind the Fostex driver. Then just add an appropriate super tweeter and go.