I have written previously on the integration between SuccessFactors an SAP HCM  and pointed out a short video that gave a high level introduction to the topic

Further information about the HCM  SuccessFactors hybrid  integration can be found on the SAP Community Network under hybrid solutions. A hybrid solution combines the best of both worlds: on-premise and cloud. These hybrid scenarios  leverages the functions provided by SAP ERP HCM and SuccessFactors Solutions to offer a comprehensive, fully-integrated solution that supports every stage of the process described in the scenario

The following scenarios are covered:

SAP ERP HCM and SuccessFactors Talent Solutions

• Basic Employee Data
• Compensation
• Recruiting
• Workforce Analytics

SAP ERP  and SuccessFactors Employee Central

• Cost Center Data
• Employee Data
• Organizational Data
• Payroll

I found a lot of helpful  information in this site, particularly the prerequisites,

Hybrid Solutions Link

If you are interested in deploying the hybrid solution quickly please have a took at the Rapid Deployment Solution