To add a user 'psk_identity' in @file{keys.psk} for use with GnuTLS run:
@example
$ ./psktool -u psk_identity -p keys.psk
Generating a random key for user 'psk_identity'
Key stored to keys.psk
$ cat keys.psk
psk_identity:88f3824b3e5659f52d00e959bacab954b6540344
$
@end example

This command will create @file{keys.psk} if it does not exist
and will add user 'psk_identity'.