append to the current search_path

How can I append to the current search_path?
How can I put $user (back) into the search path?
These commands all fail:
set search_path $user,public,gnova;
set search_path \$user,public,gnova;
set search_path '$user',public,gnova;

Ideally, I would like something like PATH=${PATH}:/usr/local/bin
where I can get the current value and just append to it.

TJ