Darren,
just import json into Swagger Editor http://editor.swagger.io/#/ via File >> Import File ...
... and download it as yaml via File >> Save as YAML.
See also:
http://bidoc.canlab.ibm.com:9080/support/knowledgecenter/SSFPJS_8.6.0/com.ibm.wbpm.wle.editor.doc/topics/int_ext_services.html?lang=en
Werner Fuehrich, Hybrid Cloud, BPM SWAT