Ok, that sounds good. So what's the problem or the question?
I don't know nothing about OpenTaps ERP, but I assume that it expose some kind of API that you can use for integration purposes. If that's the case you just need to use those APIs inside WorkItemHandlers or your application to integration with jBPM. There shoudn't be anything specific (besides the Opentaps ERP APIs) that you need to know to achieve the integration..