I have not personally used PACT but I have used the webMethods Unit Test Framework and you can create JUnit test cases using that framework: https://docs.webmethods.io/on-premises/webmethods-designer/en/11.1.0/webhelp/index.html#page/sdf-webhelp%2Futf.javaunittest.html%23wwconnect_header
Perhaps you could use the webMethods Unit Test Framework as a bridge to implement PACT contract testing.
HTH,
Percio
https://nibl.tech