How to add units on skirmish?

[gorillex]

Hi, i've wanted to play a LAN game with a friend in skirmish, but we would like just to add some units at the begining of the game, like have a Jagdtiger or something, but i don't know how to do that, just to add neutral tank ready for use. can someone explain me how to do it? i'm really not good with the gem editor, used only once and when i loaded the map i created, the game crashed on loading user interface, i need some help : / thanks!