Which optical library are you using? If it’s an IBM 3995 / 3996 (which is really a Plasmon G Series UDO library) or an HP SureStor, or a DISC jukebox then this is fairly easy. Firstly, IBM discontinued support for the UDO G Series 3995 and 3996 machines and HP has long since done the same for their machines.
If the data is residing on the library you will need a RAID appliance in between the library and the AS/400. We’ve done a few hundred of these migrations using out Hybrid Optical Storage solution. Send me an email and I’ll be glad to provide some additional technical details that will help you talk to the AS/400 directly.