You can connect a USB hard drive to the Oppo. The files have to be in wav or flac. You can convert Apple lossless to flac using the batch converter in dBpoweramp. I would suggest the 103 rather than the 93 for this, as the 103 has a better interface than the 93. The 103 allows you to select by genre, arist, or album, which the 93 does not do. But the interface on the 103 is by no means as flexible as in iTunes or one of the other computer based players.
You may be better off with a PC or Mac using iTunes or some other player and sending the signal out the USB to a USB to S/PDIF converter and sending that into the receiver. Some receivers have usb input, so you can skip the converter. You can control the music computer from a laptop or tablet or phone, depending on what player you use.
The Airport Express is also a very good option. It has optical digital out that can go to the receiver. You can also get analog out from it, but the receiver probably has a better DAC than the Airport Express. With the AE, the music files stay on the laptop and are streamed wirelessesly to the AE and then out the optical cable to the receiver. He can then use the same player software on the iMac that he has been using. This may be the best option.
Depending on which Sonus Faber speakers he has you might want to be someone selective about the receiver. A $500 entry level model is probably no going to do justice to them.