Actually, this is an extention of the cheat I published earlier. Before you can copy & paste units and buildings, you have to be able to select them. You might have noticed that you can't select everything - not enemy units nor unfinished units/buildings.
Here is a really powerful trick: press the tilde/backtick button [~] to open the console, type "ui_selectanything", press enter then press the tilde/backtick button again to close the console. Now you can select anything!
Start building a building
Immediately copy & paste
Now you have a finished building!
This also works for experimental units! Start building a Monkeylord, copy & paste it, and now you have a fully finished Monkeylord within seconds.