How to deploy SSIS package with XML config, using SQL agent
If you want to store the configuration file and/or the file you need to "pick up" via the SSIS package on a UNC path, SQL Server is going to need permissions to that direct UNC path. This permission is based on your service account being used for SQL Server Agent. If you are in a domain environment, then it is best to have your service run as a domain account and it will allow you to easily provide access to network resources.

If this is a configuration that is meant to change often and I would not be the one to do it necessarily every time, then I might chose to store the configuration in SQL Server. There are a few examples out there online that show how to do this, just one example here.