Hi, Yes, it is possible in Pega. It is a common requirement from most of the client "Reconciliation Process" - compare two system data, for mismatch data create a case in Pega case management, and assign the cases in business defined work queue.

1.First decide how you are going to pull the data from an external system, is it service or external database connectivity 

2.Once Pega has access to an external system, build a business logic/rules for data comparison, and display based on your requirement.