usermod --password `openssl passwd PASSWORD_TO_USE` USERNAME
The problem is that DB2 doesn't like SHA-512 hashes, but MD5.
On Ubuntu you can also change /etc/pam.d/common-password and change line
password [success=1 default=ignore] pam_unix.so obscure sha512
password [success=1 default=ignore] pam_unix.so obscure md5
Now changing the password will use MD5 hashes instead of SHA-512. This is a bit unsecure, so you decide if you need it. usermod should do the trick anyway.
Just remember that DB2 expects username on Linux (probably on every UNIX) to be all lowercase.