Is there a reason you need 3.6 specifically? You could run the latest 3.6 version of the editor, use Import Data to bring in your 3.6 data, save the project, then open the project with the latest, eg 3.8, and export 3.8 data.
Are you familiar with Java? The code for 3.6 is here:
spine-runtimes/spine-libgdx/spine-skeletonviewer at 3.6
If you really must use the 3.6 Skeleton Viewer I can make a JAR for you, but it would be better to use 3.8 if possible.
