i think you'll have to do it with a trigger, like unit - play unit's "work" animation or something like that.
you can watch the possile animations in the WE, open the object editor and select the building you want, search the model file (graphic - model, something around there) and open it as you do to replace it or select another one. If you do this, the building will be shown in a small preview window on the left.. go there and click the arrows above it to change the animations, and look at the names.. if you find the animation you want (i think it's something like "work") just copy the name and use it in the trigger.. should work propably
