In my project, I have an TInputDirWizardPage where user can select a directory for a database backup. So it can be any drive. I need to add NETWORK SERVICE account to the user list of this directory and modify it.
Is it possible to set this in [Dir] section? If not how can I implement this?

Yes, the
[Dirs]section can set permissions for the "networkservice". UsePermissionsflag withnetworkservice-prefixed value: