[Foodstamp]

You can mod boots of the planes to construction level 6. Then give each of the commanders the item via mod command.


(The following may or may not work)
Alternately, you could create a teleport spell that requires nature 0, research level 0, then modify the units who do not have nature magic with #magicskill #(Mask for nature) and then #magicboost (Nature Mask) -1. I think the mages would be able to cast it then, but I am not sure so you would need to test it out.