Performing a script on the server via PSoS step is like running a scheduled script on the server. So, a script running on server cannot import from a FileMaker file. You can go through the following link for more info.
Initially, I was also thinking that PSoS is different than running a schedule on FM Server. But, the only difference is that in a scheduled script, the script gets triggered automatically and in PSoS, the script is triggered by the client. In both the cases, the script is run on the server like a scheduled script.