A cursory glance suggests the answer to your question may already be in the Debian Wiki:
Ordinarily, the resolv.conf(5) file is managed dynamically by various network service daemons. This is the default, and is intended for laptops and other highly mobile systems which may connect to different networks. It also works well for many desktop and server systems, so long as the network infrastructure is perfect.
For some systems, on some networks, the system administrator may wish to configure /etc/resolv.conf by hand. This leads to a conflict between the administrator's changes, and the dynamic changes performed by the network service daemons.
If you edit resolv.conf by hand, and need to stop daemons from overwriting your changes, this page documents some of your options.
https://wiki.debian.org/resolv.conf
As with all questions, some context would be useful: why do you want or need to manually edit this file, what is your setup, and what do you hope to achieve by the addition that is nor happening already?