There are a number of standalone Lightpipe and s/pdif interfaces available, the behringer ADA8000 being probably the lowest price lightpipe interface. (you get what you pay for)
I've got a few listed on
a help page on my website, though some of them are input only devices.
Not sure that there will be any benefit sonicly of mixing outside the box into a Mackie, though, and especially not with an inexpensive interface.