This isn't really the place to put it, as this is only for engine feature requests. Such things are up to the modder, and have slight disadvantages in the AI being unable to properly use them, but have been used in some mods and tested extensively in the past. Generally, they produce a size zero unit (which effectively cannot be selected or seen) and give it a negative resource cost in the resource you wish to give, and a positive resource cost in what you take away; eg, -100 wood, +100 gold would trade gold for wood). GAE is also able to generate without creating a new unit, but this feature is not (yet) available in MG.
