Short answer, it is not possible. The listener for this is only viable on SFMC Import folder and those subdirectories below it.
Now, it is possible to 'fake' this, but it requires your 3rd party system to drop the main file on your external FTP and then drop an empty or small file to SFMC SFTP folder when that file is finished being uploaded.
- Set up a Triggered Automation (select your desired folder and file name pattern for the 'dummy' file)
- Set your import activity to pull from the external FTP to grab the
- Create the activity in your 3rd party data system to first drop the data file to your external FTP.
- After this file has finished loading, have it shoot over a dummy file to the SFMC to trigger the automation.
Another option if your system can send API calls is to:
Have a 'scheduled' automation set up with the import activity but do not schedule it.
You then have your external system shoot over an Automation Studio
API call to start the automation once the file has finished uploading
to the FTP.
- Your Automation runs, importing the file from the external FTP.
Both of these are a bit much and require a good amount of extra work. Unless it is necessary to store this file elsewhere, I would adjust the process to drop it to SFMC instead.