You add an entry to the parameter map see sample code below:
var parameterMap = qie.newParameterMap();
parameterMap.put("http.header.content-type", 'application/xml');
parameterMap.put("http.header.headerName", 'someValue');
var urlTemplate = qie.evaluateTemplate(qie.getWsEndpointUrl('wsName'));
jsonResponse = qie.callRESTWebService(
"wsName",
urlTemplate,
"GET",
null,
parameterMap,
60000);
See also:
Handling HTTP requests and responses
Making Requests via qie.callRESTWebService()