I don't think I understand why you need to do this.... I thought that receiver was able to output a digital input to zone 2 as long as it was the zone 1 selected input? I wouldn't recommend looping the channels back into the receiver through a line-level converter, for many reasons. Beyond the problems with loosing channel information, it links the two zone's volumes.
That said, if your receiver doesn't downmix the zone 1 digital input to zone 2 by itself, I'd recommend plugging the analog outputs of your zone 1 sources into the receiver. It's a pain, but it's the best way to do it.