I am able to add a single test case result using the /testcaseresult/create API and passing the JSON as:
 {
    "testcaseresult":
    {
        "Build":5,
        "Date":"2017-02-27T18:03:29.260Z",
        "Testcase":{"_ref":"https://rally1.rallydev.com/slm/webservice/v2.0/TestCase/12345678"},
        "Verdict":"Pass"
    }
 }
Is there a way to add multiple test case results?