After getting a few questions about how I did the Ninja B.A.T. conversion to an articulated arm I decided to try to put together a step by step "tutorial" of sorts.  So here it is.  Good luck!

What you're going to need:

2 Ninja B.A.T. figures
Dremel with cutting wheel and 2mm drill bit
Patience, lots and lots of patience

For the purposes of this "tutorial" we'll be using two different names for each of the BAT figures.  The figure that we will be using for parts will be the "DONOR" figure and the figure we're installing the new parts on will be the "FINISH" figure.  Also you need to resign yourself to the fact that you will be pretty much ruining one of these figures (the DONOR)  Got it?  Good.

Now first we're going to take our parts from the DONOR figure.  Start by using your dremel and cutting wheel to start to split the upper right arm (bicep) of the DONOR figure in half.  Its easiest if you follow the glue seam lines on the front and back of the arm.  Since the plastic is pretty hard you'll have to cut for a while to get through but you want to be sure to just cut enough to break the seam.  Once your cuts are complete you can pry open the bicep and pull out the lower arm with the joint intact.  Use this same procedure on the lower left arm to remove the BAT hand.  What you'll discover is that the joints on both the elbow of the arms and the wrists of the hands have a unique construction ( see image below )

Notice the round "cap" on the end of the peg?  That round flat piece needs to be cut off from both the elbow and wrist joints being careful to leave as much of the pegs as possible.

Also on the lower right arm of the DONOR figure (the part we just took off to be transplanted) you want to cut off the right hand at the wrist being careful not to scuff or cut the arm itself.  The round "cap" should be left inside the arm as it will give you something to drill into to fit the left hand wrist peg.  Using your dremel drill bit (I find that a 2mm bit works best) go ahead and drill into the wrist of the DONOR lower right arm that we will be transplanting to the FINISH figure.  The hole you make will be smaller than the wrist peg so grind a little bit off until the peg fits nice and tight.  You want it to be able to slide in and swivel but not be so loose that it falls out.  Mine is actually really tight since I figure it will wear down over time.

Now to the FINISH figure.  Go ahead and pull the grapple arm out all the way so the cord is exposed.  Cut off the lower left arm at the cord and put the arm to the side as you won't be needing it and push the grapple retract button on the belly of the BAT to retract the cord for the last time.  Using your dremel with the cutting wheel again, cut off the "peg" extending from the upper left arm (see picture below)

Use your dremel and drill bit and drill a hole in the FINISH figure left upper arm where the hole for the cord is already started for you.  Drill through as far as you can so the arm elbow peg will go in all the way.  With your hole drilled follow the same procedure you used for the wrist hole and grind it down til the elbow peg fits.

Once you get the hole drilled go ahead and slip the elbow peg in and there you have it!  A Ninja B.A.T. without the lame grapple arm feature and a new point of articulation!  Oh and a torn up Ninja B.A.T. that you can use as a "Battle Damaged B.A.T." or something.