Without using another audio recording or computer interface device, I suppose you would have to dedicate a channel on the X1800 to putting the audio back into the mixer and then use the discrete USB audio of that channel back into the computer.
Perhaps the DVS function could also somehow be used for that while still using the channel, but I’m not sure if you can override the phono input capability that gets routed to the computer, or still make use of the SPDIF or line input of the channel that I think might become dedicated to USB input from that channel to the X1800 mix bus. Starts to get very confusing to think about.
I’d suggest that if you’re sending the X1800 into another device and want to record that other device, you don’t try to record that second device using the X1800 but rather get a third recorder or audio interface to go into.
I’m not sure what this second device in the chain is you’re talking about, but you also might want to consider just using it as an effects unit using the FX send/receive on the X1800.