It's not really an option from the installer, AFAIK. However, it is still
possible. Use a "file" URI in source.list(.d) and mount the NFS before
preforming apt operations.
From the sources.list(5) manpage:
URI SPECIFICATION
The currently recognized URI types are cdrom, file, http, ftp, copy,
ssh, rsh.
file
The file scheme allows an arbitrary directory in the file system to
be considered an archive. This is useful for NFS mounts and local
mirrors or archives.
The debian installer should only run on a single virtual terminal. The should
be at least one or two "recovery" consoles allocated that will allow you to do
the NFS mount before proceeding further in the installer. You can switch
virtual terminals with Ctrl+F{1..6}, unless you are in an X11 session, in
which case you'll need to use Ctrl+Shift+F{1..8}.