I think that you can implement this behavior in different ways in APS:
- Adding a custom REST Task for callling an external service via REST: https://docs.alfresco.com/process-services/latest/using/process/bpmn/#rest-call-task. Your external service will be called with a POST call and a JSON body in the request where your service will get values to store on a different data source (MongoDB, other DBMS, file system and so on)
- Using Custom Data Models: https://docs.alfresco.com/process-services/1.9/develop/dev-ext/#custom-data-models
- Implementing your own Service Task as a Java Delegate with your own DAO classes, and here I suggest to use the Spring Framework using Spring Data or Spring JdbcTemplate: https://docs.alfresco.com/process-services/latest/develop/dev-ext/#custom-logic
- Implementing a Task Execution Listener attached to the end event for example with a similar implementation of the Service Task
- Remember that, if you are using a recent version, you can disable the persistence of some variables in the Activiti database: https://docs.alfresco.com/process-services/latest/using/process/persistence/
Hope this helps 
