I got a X.509 (S/MIME) certificate from GlobalSign. I imported it into Kleopatra, but I cannot associate it with an identity.

i use the same approach as mentioned in this thread:
http://readlist.com/lists/suse.com/suse ... 22678.html

But the options to change the trust level of that certificate are all blanked out (see linked image).
http://img37.imageshack.us/img37/430/kmail2x509cert.png

How can I get kmail2 to use the certificate?

Have you trusted the root certificate? It may be required in order to trust certificates which the root certifies.

Thx, You got the solution.

Put this into a text file
https://www.globalsign.com/support/root ... alsign.php
imported into Kleopatra and could choose "Trust Root certificate". Now it allows me to choose my own certificate for encryption and signing (don't have to explicitly trust my own certificate).