I'm fairly certain that the actors were able to do that because the A-frame was "wilded" over to where the shuttlebay set was located - there's certainly no way it could fit in Stage 8 with regard to where the A-frames are usually located. Without the usual constraints of the corridor walls, the actors could enter in from anywhere they pleased!
It also shows us which A-frame was used, since the one by the Transporter Room (http://tos.trekcore.com/hd/albums/1x...timehd0537.jpg
) has a red alert panel to the left (as you look at it). The one in JTB (http://tos.trekcore.com/hd/albums/2x...abelhd0086.jpg
) is more likely the one opposite Engineering, near the Sickbay lab.