Given a CA certificate file 'foo.crt', follow these steps to install it on Debian
Copy your certificate /usr/local/share/ca-certificates/
and then, update certificate store:
update-ca-certificates
You should get this output:
Updating certificates in /etc/ssl/certs... 1 added, 0 removed; done. Running hooks in /etc/ca-certificates/update.d.... Adding debian:foo.pem done. done.