Like, there will be a list of available units on left panel, we click and click to create it. Cost, time they will remain same
Is it possible in custom game?
It is possible, but you have to use the Advanced Genie Editor (A.G.E. for short). This video explains how to do it. You can use any unit, from heroes (example) to cheat units. Above all else, DO NOT CHANGE THE BASE FILES–create a copy and use that as the video explains, or you will break the game. Let me know if you have any questions!