as soon as you have the possibility to choose between different digital outputs, e.g. from your onboard sound card, you can route the audio signal from MIR with a optical cable to cubase (as an external instrument or audio input) and make your final mix there. afaik there is no software way yet to solve this problem except if your sound drivers allow internal routing.
I use a RME HDSPe AIO card and need to do the "optical cable trick" too. There i use the ADAT output channels 1 and 2 in MIR and ADAT input channels 1 and 2 in Cubase. Works fine.