RQM API - Adding testcase link to testplan

I am able to PUT the links with a request that looks like the following.  The problem is that this request replaces the test cases that are already defined in the test plan.  Is there an easy way to simply ADD a test case link.

<?xml version="1.0" encoding="UTF-8"?>
<ns15:testplan xmlns:ns15="http://jazz.net/xmlns/alm/qm/v0.1/"
    xmlns:ns2="http://schema.ibm.com/vega/2008/" xmlns:ns3="http://purl.org/dc/elements/1.1/"
    xmlns:ns4="http://jazz.net/xmlns/prod/jazz/process/0.6/" xmlns:ns5="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
    xmlns:ns6="http://jazz.net/xmlns/alm/v0.1/" xmlns:ns7="http://purl.org/dc/terms/"
    xmlns:ns8="http://jazz.net/xmlns/alm/qm/v0.1/tsl/v0.1/" xmlns:ns9="http://jazz.net/xmlns/alm/qm/v0.1/testscript/v0.1/"
    xmlns:ns10="http://jazz.net/xmlns/alm/qm/qmadapter/task/v0.1"
    xmlns:ns11="http://jazz.net/xmlns/alm/qm/qmadapter/v0.1" xmlns:ns12="http://jazz.net/xmlns/alm/qm/v0.1/executionworkitem/v0.1"
    xmlns:ns13="http://jazz.net/xmlns/alm/qm/v0.1/executionresult/v0.1"
    xmlns:ns14="http://jazz.net/xmlns/alm/qm/v0.1/catalog/v0.1" xmlns:ns16="http://jazz.net/xmlns/alm/qm/styleinfo/v0.1/"
    xmlns:ns17="http://www.w3.org/1999/XSL/Transform">
    <ns15:testcase href="https://jazzc01.hursley.ibm.com:9443/qm/service/com.ibm.rqm.integration.service.IIntegrationService/resources/IBM+Ad+Hoc/testcase/urn:com.ibm.rqm:testcase:178652" />
</ns15:testplan>