Can I display data from an outside source or outside out-of-the-box field history tracking into a Process View?

Yes, you can import this data into the Event Log (hbpa__Event_Log__c) object with 5 required fields:

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Record Id: hbpa__Case_ID__c
New Value: hbpa__Activity__c
DateTime: hbpa__Timestamp__c

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Process ID: hbpa__Process_Definition__c
View ID: hbpa__Batch_ID__c

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Other optional fields include resource making the change, source system, external id, and other dimensions to filter by.

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