Is it possible to add a TCER using RestAPI for the existing test cases and test plans using RestAPI ?
One answer
Yes, using both the RQM Reportable REST API and RQM OSLC API. Using the RQM Reportable REST API , you could create (POST or PUT) a new executionworkitem resource, referencing the test plan, case, etc. (see https://jazz.net/products/rational-quality-manager/api-doc-6.0.3/schemas/qm_xsd/complexTypes/abstract-executionworkitem.html). See https://jazz.net/wiki/bin/view/Main/RqmApi#Create to get started.