5.1 surround sound external sound card for iMac Pro

not really. there is sorta some help for using that 7.1 usb external soundcard on a Mac (dr-lex.be) but it only works (kinda) up to Sierra. Creative makes something that sorta works with the Mac, but I've never used one with my Mac, so can't say 100% for sure if it does or not; or even how well it does. Your best bet, apart from looking at Creative, which is iffy, would be to get a USB to S/PDIF or coax adapter and use that for surround sound, at least digital surround sound. You'd also need a USB-C to USB B cable to connect to your iMac Pro 2017, and the S/PDIF cable to connect to your Z906 speakers. After you buy the adapter and cable(s) , and hook it all up, under system preferences/sound, you'd need to specify USB audio output. then give it a try. I have the same speakers as you do (love 'em) and the digital input would be 2 on the control box. You'd also have to make sure that the button on the control box is set to 4.1 or so. Please note that the only way to get surround sound from any mac is either via HDMI or digital audio, and only with certain movies that have been surround encoded. VLC (videolan client) and AppleDVD do support surround sound, as does iTunes (on occasion) but you'll have to assume that unless it says it, it'll be in stereo, period.

maybe Soundflower ??? About the last sound card I knew that worked with Mac was Griffin's fire Wave card, but that's ancient history.

well, that's all I have

john b