If you want to add a certificate into a "cert8.db" file from a certificate file, you can use the Mozilla "certutil -A -i" command as shown in this tutorial:

C:\fyicerter>\fyicerter\nss\bin\certutil -A -d .\
-i GeoTrust.pem -n "GeoTrust SSL CA" -t ",,"

C:\fyicerter>\fyicerter\nss\bin\certutil -L -d .\

GeoTrust SSL CA                                              ,,

What you are getting from this tutorial:

• "-A" command adds a certificate to the certificate database.
• "-d .\" specifies the directory where the database, cert8.db, is located.
• "-i GeoTrust.pem" specifies the input certificate file.
• '-n "GeoTrust SSL CA"' specifies a name for this certificate.
• '-t ",,"' specifies trust arguments for 3 categories: SSL, email, and object signing. ",," means no argument is provided. So it should not be trusted.