Thanks for the quick answer.
If I understand well, there is not a real way to set the unused IO of the adapter (as output) and make a measurement using the impedance instrument and make that the set IO remain in their state. It is one or the other.
Actually I was able, using script to make link between 2 Discover2 (one for the impedance, one for the multiplexer):
use DIO0-3 to talk to the Discover2 with the mux and set channel
run impedance instrument and get measurement
This way the channel is latched by the second Discovery2 attached to the mux