Can a pilot act as cabin crew member?
For the U.S., can a non-cabin crew act as a cabin crew? Yes, except for safety duties.


  U.S. air carriers periodically use company employees in the cabins of its aircraft for the purpose of conducting certain passenger service activities, such as serving beverages, conducting customer relations, or acting as translators. These persons are not assigned to flights to perform safety duties. These company employees are not acting in the capacity of an F/A nor are they, in general, trained or qualified to act as a F/A. The regulations do not prohibit the use of non-F/A personnel by an air carrier. However, their presence could conceivably interfere with the F/As if they were not properly instructed.


Source: FAA

My interpretation is technically yes, especially that pilots are trained in safety duties.

But that's not to say there wouldn't be a union rule against it.