You don't. You can confer permissions to local resources for the IIS APPPOOL{app pool name} identity for local resources per:
How to assign permissions to ApplicationPoolIdentity account
In Active Directory, the identity needs to be either a Well-Known security principal, an actual user/group/computer security principal, or a foreign/trusted security principal.
However, if you use the Network Service identity on the IIS AppPool, the application pool will use the machine account of the IIS server when accessing network resources. In that case, you can confer the necessary permissions to the computer account (domain\computername$) in Active Directory.
http://www.iis.net/learn/manage/configuring-security/application-pool-identities