You would certainly have your work cut out attempting this, but anything is possible.
Converting a JBPM class to a Pega class would be relatively easy compared to converting a JBPM process diagram to a Pega Flow rule.
Start by looking at the pzAddDefaultStagesStartingFlow Activity.
Using Referencing Rules you will find that this activity is called by pzAddDefaultStagesStartingFlow which is called by pxCreateCaseTypeStagesAndFlow which is called by Work- pxCreateTopCasetype which is called when you click "Add Case type" in Dev Studio, or click "New" in App Studio, when inside the Cases types view.
Note the use of the pxUpdateRecord which is part of the Engine's "Records API" which was started in Pega 7.4. Part of the Records API's responsibility is to choose the most appropriate ruleset for a rule which is especially important when App Studio is used.