You can't make those sorts of changes in Mission Builder but you can hand edit the mission to reduce the AI altitude and speed to anything you want, including 0/0. However, if you do that they probably will crash at the start of the mission.
For example, if you create a flight of AI bombers then edit the starting altitude to zero (ground level) and the speed to zero (or 10 kph) they wont start out taxiing along the ground. Since they were originally flying when you wrote the mission they will start with their gear up and crash as soon as the mission starts.
With regards to the B24 Guy's folded wing AI, if they were modeled with folding wings wouldn't they stay that way? Even if you could do what you want you would end up with planes with folded wings flying around.