Then, I could import into Thunderbird using the Import menu in the certificate manager. I finally managed to figure it out. Now I am able to digitally sign my email messages using the S/MIME signature. The Certificate Authority was used to issue the certificate is CaCert a.k.a. 'Root CA' in the keystore.
This means that the emails I will send digitally signed will make sure my identity is verified and the message's content free from tampering or alteration while it reaches the recipient. The link on superuser.com contains my contribution to the community in relation to the matter.
Happy email signing and stay secure!