How to add a new Trade item into trade list?

[Shaqulli]

My goal is adding a trade item called Unique Unit into trade list.I feel diffcult about it;I tried copy a INstance ,and changed its Name then paste in the DiploTrade.xml.Failed of course.
First,I hope the Item "Unique Unit" is a drop-down list that contains all UU the civ has.
It will appear in trade list if both you have the tech that unlock the UU,and you don't have the tech that obsolete the UU.
Second, it can be purchased after Do Friend.
You can choose the quantity you buy，just like strategy resource.
The price per unit is the same as the upgrade cost——Gold equals to usually 25% of the unit production cost.
By the way,AI will think it worthless if he has bought 6 or more.
Third,after you have bought 6 unique units.You can convert 6 units(have the same unitclass with the UU you bought) to the UU you bought.Like this picture. The ''Qin Empire" Mod provide a panel to convert unit into UU.Actualy,he kills the unit and creat a UU,then place it.I'll ask permission to borrow his panel.This is ''Qin Empire" Mod.Now who can help me with the first step?

 

[Shaqulli]

I have make a button to convert my unit into unit.Thank to help from Captain_CW,the authour of Mod "Qin Empire".He is skilled and helpful.
And I notice that the Mod "Super Power：complete edition" has defined some new resources(such as manpower),and allows leaders to trade these resources.I hope I can find method from it.