The arm joints were taken from a Skeleton Warriors Grimskull (?) figure because the elbow was correctly sized.

Also, the rotating forearm at the glove line on Grimskull looked like it would match up with the glove line on Batman. When swapping limbs, I always try to take advantage of lines or contours in the figures that will hide the seam.

First I cut the elbow out of the arm on each figure, and added the articulated Grimskull elbow to the Batman figure. Again, careful comparison and measurements need to be made before cutting.

The Grimskull forearm rotates using a peg and socket connection, which you can see disassembled here.

Here is another view showing the trimmed down Grimskull glove.

The Batman arm was also trimmed down to the glove line, as indicated by the white lines.

Next, I attached the forearm peg to the elbow. I would never had removed it, but the arm would have been too long otherwise.

Here is where it gets tricky. Keep in mind that I am trying to transfer the rotating forearm feature from Grimskull to Batman. I cut the peg socket out of the Grimskull glove, and drilled a hole in the Batman glove large enough to accomodate it.

Then I inserted the Grimskull socket into the Batman glove and glued it into place. I also trimmed down the peg to make re-assembly easier.

Here is the completely assembled arm with both inboard...

...and outboard rotation. The elbow works as well.