sudo mkdir -p /etc/skel/whatever
/etc/skel is the "skeleton" of a new user's home directory; anything located therein is replicated as the starting point for new users' homes.
From, for instance, the manual page for
-k, --skel SKEL_DIR
The skeleton directory, which contains files and directories to be
copied in the user's home directory, when the home directory is
created by useradd.
This option is only valid if the -m (or --create-home) option is
If this option is not set, the skeleton directory is defined by the
SKEL variable in /etc/default/useradd or, by default, /etc/skel.
If possible, the ACLs and extended attributes are copied.
adduser will copy files from SKEL into the home directory and prompt
for finger (gecos) information and a password. The gecos may also be
set with the --gecos option. With the --disabled-login option, the
account will be created but will be disabled until a password is set.
The --disabled-password option will not set a password, but login is
still possible (for example with SSH RSA keys). To set up an encrypted
home directory for the new user, add the --encrypt-home option. For
more information, refer to the -b option of ecryptfs-setup-private(1).