is there anything exist to convert xml -> yaml directly?
The difference between XML and YAML is significant enough to warrant a redesign of the schema you are using to store your data. You should write a script to parse your XML records and output YAML formatted data.
There are some methods out there to convert any generic XML into YAML, but the results are far less usable than a method designed specifically for your schema.
I think you want to look at the YAXML draft spec, which describes how to represent YAML data structures in XML.