i am fairly sure it cant be changed. if it is very important (and worthy of some work) why dont you make some little pictures of the units (the ones you see when you click on the zeplin) and change them to have number ranged the amount you expect to be boarded. then you can set each one to a unit. then do a trigger that uses unit groups, so when each unit is added to the zeplin it changes the picture up one unit. then when it unloads, depending on the unit in the zeplin a certain amount of units wil be offloaded. this is alot of work and has downfalls. it would only work for 8 different units and it wouldnt work for heros. i would think it better if when a unit is loaded then get transported to an unseen part of the map and the picture comes up in the zeplin for only 1 unit. tho again limits to 8 units. or you could just use a double zepplin thing