On a Windows system you have a 2-step solution:
1. Create an "opener file" - that is, a new, blank Filemaker file and create a new script that opens the import script in question in the other file. You will need to add a file reference to do this. Choose File...File Options and under When Opening this File check Perform Script. Select the 'Opener Script' you just made.
When this file is opened, it triggers the script in the other file to import. That leads us to Step 2 - Scheduling
2. In Windows, Click Start..Control Panel...Scheduled Tasks...Add a New Task
When it asks you for the Program you want to run, click Browse and Select the Filemaker file you just created. Set your schedule and it should work.