There are five Reserved Groups in this mission (let's call them A, B, C, D and E). The events that trigger the deployment of A and B are straightforward. But there's a conditional trigger for C, D and E and it will only trigger one of those three groups and not the other two. That condition is determined by the state of the campaign and that state is the same throughout the mission, so as the Imperial player, I know in advance for our mission that it will be C, and not D or E.

So knowing this condition in advance, I would like to add D to my open groups during setup so I can deploy them at my own threat expense. Can I do this?

no, once it’s in the reserved group, it can’t be deployed unless mission rules allow.

Reserved Groups
· Mission rules specify when and how these figures can be
deployed to the map.

Reserved groups can only be deployed by mission rules, that's why they are called reserved. If an event doesn't happen, or an event doesn't deploy a reserved group, then that group cannot be deployed.

Also, deployment cards for reserved groups are taken first, so you don't have the card available to add to open groups even when you know for certain that it won't be deployed by an event.

That makes sense. Thanks.