I don't have any personal experience with System Center, but WFA does have a web services WSDL that any orchestrator can leverage. We've done some integration work with customers for HPOO, and BMC Atrium Orchestrator, as well as some custom portal integration. With WFA 1.1.1 SOAP is available, and WFA 2.0 due out at the end of this year REST is being added. If you'd like to read up a bit more on what is available, please check out the Web Services Primer document HERE.
Hope this helps,