Hey Marek,
Well, I don't know that you need anything additionally to backup webshpere. But I do know that for the DB2 agent, you will need to install that in addition to the AIX netbackup software. You will need to make appropriate changes to configuration files of course, if you want to run the backup automatically.
Just to recap, the AIX netbackup software will hit file system and open file backups, but you will need to install the DB2 agent as well in order to backup the database... Hope this helps.
Alex