Petra - interpretation can be exported to Petra as so called "repeated tops". It is a special file format that can be then imported to Petra. To export "repeated tops", right mouse button click on interpretation-> export. In new window select "Petra repeated tops" in "save as type" line. Then save the file.
Now you can import it to Petra.
Petrel - you need to download Petrel plugin from here (free of charge) : https://www.ocean.slb.com/en/plug-ins/plugindetails?ProductId=PAGS-B1
After that you can open StarSteer projects in Petrel and transfer any data from those projects to Petrel with one button click. All operations are done directly in Petrel. There is a user manual link at the same web-page.