Before going digital, I would often track with a touch of reverb (usually room verb) one each part and then add a different reverb (to the entire mix) at mixdown. Now, I almost always do it all "in the box."
To use outboard gear during mixdown, you will need both excellent D/A and A/D converters as well as outboard reverbs that are significantly better than your software verbs. Otherwise, it's just not worth it. The signal loss of the routing out and then back in must be outweighed by the quality of the reverb.
Others may be able to better describe their techniques for doing this.
I say, it may be better to buy some good software verbs and sell your outboard verbs to the analog guys.