--gecos option to skip the
chfn interactive part.
adduser --disabled-password --gecos "" username
It's all in the man page. Not the most obvious formulation tho.
Set the gecos field for the new entry generated. adduser will
not ask for finger information if this option is given.
The GECOS field is a comma separated list as such:
Full name,Room number,Work phone,Home phone, despite that man page mentions
finger information Details - Wikipedia
Hope this helps you.
Update: for more recent (or upcoming) versions of
--gecos option is replaced by
Set the comment field for the new entry generated. adduser will
not ask for the information if this option is given. This field
is also known under the name GECOS field and contains informa‐
tion that is used by the finger command. This used to be the
--gecos option, which is deprecated and will be removed after
Debian bookworm. Valid Modes: adduser, adduser --system.