There are a number of DVD burners available for the iSeries, you will probably need a DVD-RAM drive or multi-recorder for backup purposes.
As far as I know none of the normal IBM backup procedures supports backup to optical media – you can however write your own routines to handle this. For example, you could save your data to a save file and burn the save file to a DVD. You will also have to write your own routines to handle restores from optical media. Alternatively you can try and find a third party product which will let you save/restore using optical media.
The lack of support for this is one of the reasons that my site still uses tape for backup. We have to provide data on DVD to a third party, and we ended up transferring the data to a PC and burning the data to the DVD from the PC.
Maybe you’d be better advised to get a new (more reliable) tape backup.