I’m going to take a shot here, but don’t bank on this. Someone with the IBM system congfigurator software should really do this. Don’t try this unless you are willing to return to the original configuration and do a complete reload.
Do a full(option 21) save to tape. I’d do two fulls myself. Verify your alternate IPL device is set to your tape drive(s). Set your system in manual mode and boot to DST. Stop raid. I’m reasonably sure you cannot have different size drives in the same raid set but you should be able to end raid, then add the two new drives to the ASP and mirror them as the load source, then start raid on the other drives and restore your system and data from tape. I know I’ve seen this configuration I just can’t tell you all of the exact menu options without the screens in front of me.
If you don’t have a business partner that can help with this, try it and if it doesn’t work you can always remove the new ones from the ASP and restore from tape.