A shell script for Automator that can convert .plist to .json file.

plist-to-json.sh

for f in "$@"
do
	filename="${f%.*}"
	plutil -convert json "$filename".plist -o "$filename".json
done

Usage

1. Open Automator and create a new workflow with Service type.
2. Choose Run Shell Script.
3. Copy this gist to the code field.
4. Save the workflow. The name of the workflow is up to you, it will become the service name.

Now, just choose a .json file in Finder, and pop up the context menu, choose Convert to json (or other name you had just gave), then there would be a .plist file.