I wonder if I could develop the business logic of a powerapp through Windows Workflow Foundation or some other bpm? Similar to Appian's Low Code and Appian's BPM, which are integrated.
Not directly - there are no connectors that would link PowerApps with WF. The main "workflow" engine in the PowerApps environment is Microsoft Flow, for which there is a very tight integration.
Notice, however, that if your (existing) workflow can be exposed as a REST API (either directly, or via another custom API), then you should be able to consume it - take a look at the Custom API tutorial for more information.