There is not a way for the editor to combine skeleton data files, sorry. The skeleton attachment feature that has not been released would also not combine multiple skeleton data files into one.
You could write a shell script, for example this is a bash script:
http://n4te.com/x/8833-55hB.txt
The result:
http://esotericsoftware.com/demos/exports/demos.json
Or you could use existing formats that can store multiple files, eg ZIP, which would also work for binary. Deflate compression (used by ZIP) works well on both JSON and binary.
Or, for each binary file you could write a length, then the data.
If you are using spine-ts, the skeleton data, atlas, and image files can be embedded in the HTML using data URIs.
Note using JSON is not suggested for most use cases:
Export - Spine User Guide: JSON