It is possible to 'Proxy record' the SOAPUI traffic. This involves configuring SOAPUI to send its traffic to the server via the Silk Performer recorder.

You can configure this with the following steps:
In Silk Performer, confirm the HTTP port via Settings | System | Recorder | Proxies. By default its 8080.

Open SOAPUI and go to File | Preferences | Proxy settings. Enter localhost into the Host field and 8080 into the port field.

Save those settings and go back to Silk Performer.
Start the recorder with the following method - Record | Recorder (Do not use the Model Script option).

Now run your test case in SOAPUI.

The HTTP traffic will get routed through the Silk Performer recorder and the function count in the recorder should increase. Save the recorded script and the SOAP / XML should be scripted for you in the Silk Performer workbench.