Attachment Type Maintenance Authentication

We are trying to set up Attachment Types and want to use the File System storage type. However, we would like to be able to set the Authentication on it to our Application Pool username for writing files to the network instead of individual user. The authentication area gets grayed out when you select the File System storage type. Does anyone have any ideas how to make that happen? We did not purchase docstar and don’t want to use sharepoint, google drive, or dropbox. The goal is to not let the average user have direct access to the server and have Epicor be the control for maintaining attachments. We are on-premise so the Cloud is not an issue here.