I am looking for a way to create a Processing rule in Adobe which allows to populate entry source of an event across the visit. As a result value of this eVar needs to stay unchanged across whole the visit. Does anyone of you have experience in that?
I think this depends more on your eVar's settings. Try using "Original (First Touch)" Allocation with "Visit" expiration. Then you can set the eVar's value in your implementation code for all hits, but AA will only attribute to the first tracked value of the visit.