You can manually add a certificate to the system trust store by copying the certificate to either the /usr/share/pki/ca-trust-source/anchors/ or /etc/pki/ca-trust/source/anchors/ directories. You must run the update-ca-trust command to refresh the system trust store after you make manual updates to these directories.

For example:

sudo cp /path/to/public.cert /etc/pki/ca-trust/source/anchors
sudo update-ca-trust

See the update-ca-trust(8) manual page for more information.