Select the channels, set them to 100, select just them again and without hitting enter hit record sub then the number, then enter. Assign your sub to a fader, ghen sub-sub and select the sub and mark it inhibitive. Now it has to be up for FOH to come on.
Yes, if it's active, but even then not until you "commit" the edit. So if you're adding a bunch of channels at full to a collection to put into a submaster, if you do it in live you're going to see those channels being brought up on stage as you edit the sub. If you do it in blind, you don't see it until you complete the edit (and make the sub active). This is, after all, the point of blind; it's not happening live.