Once racked and plugged through SAS connection, the disks had to be
- Initialized and Formatted
- Include to existing raid group
- Added into existing ASP

This was done just in the correct moment... My nagios status from this morning before adding the new disks:
Storage: OK - 20.77% free
Which is just 0.77% away from a degraded iSeries (after 80% used there is a loss in performance). New nagios status now is:
Storage: OK - 68.24% free

So for some months it will be calmer... until the same situation arrives!