debian add mirror after installation

[Weijun Lu <luweijun1992@gmail.com>]

Hello.
When using the debian12 dvd disc for installation, choose to use network mirroring in the mirroring step, but at this time the system network is disconnected and the installation cannot continue.
When using the dvd installation disk, it is to complete the installation quickly, but also want to install the system and configure the mirror station for subsequent use.
1. Is there a way not to select the network mirror site in the installation wizard, and reconfigure the network mirror site through dpkg-reconfig xxx and other methods after the system installation is complete.
2. The installation wizard selects the network mirror site, but the actual installation process uses the software package in the DVD disc.