gnus-secondary-servers

gnus-secondary-servers is a variable defined in `gnus.el'.
Its value is

nil

• This variable is obsolete since 24.1;
  use `gnus-select-method' instead.

Documentation:
List of NNTP servers that the user can choose between interactively.
To make Gnus query you for a server, you have to give `gnus' a
non-numeric prefix - `C-u M-x gnus', in short.

You can customize this variable.