Yes, it's possible to add a Google Apps Script file to all spreadsheets in a folder at once using Google Apps Script.
Here are the steps you can follow:
Open the Google Drive folder that contains the spreadsheets you want to add the script to.
Open one of the spreadsheets in the folder and go to the "Tools" menu, then select "Script editor".
In the script editor, click on "File" and select "Make a copy". This will create a new copy of the script file that you can use for all the other spreadsheets in the folder.
Give the new script file a name, and then close the script editor window.
Go back to the Google Drive folder that contains the spreadsheets you want to add the script to, and select all the spreadsheets by holding down the "Shift" key while clicking on each file.
With all the spreadsheets selected, click on the "More actions" button (the three vertical dots) and select "Google Apps Script".
This will open a new script editor window for all the spreadsheets at once. In the script editor, click on "File" and select "Add a script file".
In the "Add a script file" dialog box, enter the name of the script file you created in step 3 and click on "Add".
You should now see the new script file added to all the spreadsheets in the folder. You can now modify the code in the script file to suit your needs.
Once you have finished editing the code, save the changes and close the script editor window.
That's it! The script file should now be added to all the spreadsheets in the folder, and any changes you make to the code will be automatically applied to all the spreadsheets.
Regards,