Yes the horizontal dropouts on the Xtra should work to tighten your chain.
Two concerns with the frame. First you will need to purchase an extra FAP plate since you have no chainstay bridge. Hopefully you wont have to modifiy/shorten the Xtra tongue as your clearance from the wheel to seatpost tube is tighter than most bikes.
Next is your rear spacing. It should be okay if the frame is newer. The problem if it needs to be spread it will be hard because of the aluminum. Check your Space between dropouts- should measure 128-135mm
http://xtracycle.com/forums/viewtopic.php?t=52
If it is shorter than 128 you will have to spread the frame and respace/new axle on your Nexus wheel.
I cant see your brakes, but you will have to use a Vbrakes or disc for the rear.