The CAHPS Consortium advises against adding other items to the CAHPS composite measures. All sponsors of a given CAHPS survey should use the same set of core items, which produce the composite measures, so that survey results are standardized and comparable.
Survey items are grouped into composites based on two criteria: (1) the items all relate to conceptually similar events and (2) the items have a demonstrated statistical relationship. Also, in almost all cases, the response options to questions in the composite are the same.
Adding other items to existing composite measures changes the psychometric properties of the composites--even if the items seem highly related--and poses the risks of confusing report users. In addition, a change in the structure of composites makes comparisons between different sponsors' surveys impossible.
One option is to include other items that you believe are similar to a composite measure in the same section of a public or private report. For example, a sponsor may ask health plan members questions that seem similar to CAHPS items about getting information from written materials, filling out paperwork, and getting help from customer services. Members' responses to these questions could be reported in the same section of a report.