JSON Response without unescaped format

Trying to use a Service Request Custom on orchestrator, the response always return backslash before each double quote, I think is because the HashMap just work with String at the second argument in this definition:

HashMap <String,Object> returnMap = new HashMap <String,Object> ();

returnMap.put("json", nums);

I tried to change Object to another type but return this message:

 "message" : "java.util.LinkedHashMap cannot be cast to java.lang.String",

 "exception" : "java.lang.ClassCastException"

Im working with Orchestrator Versión Studio: 6.1.4.0

Expected Result:

{

"json" : {"name":"FirstName", "DOB":1999}

}

Actual Result:

{

"json" : "{\"name\":\"FirstName\", \"DOB\":1999}"

}

Thanks for any advise