How can I add to the list of units sold mid-game?

As the game progresses, I want to be able to add new tiers of units to several of the buildings that sell them. Is there anything similiar to the Add Ability function that could do this?

The only thinkg I can think of, would be to put an upgrade requirment on the units. Such as unit requires upgrade "MidGame", preventing the user from purchasing the unit till the upgrade is researched. Then mid-game just run a trigger to reasearch that upgrade for all players.

It makes sence in my head, but I'm kind of an odd thinker.

I considered that, but then the players will know what's coming -- I'd rather it be a surprise.

Use the trigger "Neutral Building - add unit to market place"

Thanks Obsidian

For anyone else that happens to miss the gray text at the bottom of the selection windows -- make sure that whatever you're using this on has the Sell Units ability.