How to make neutral units, which will not disappear in melee games?

For example eggs and Arbiters in Triathlon: http://www.teamliquid.net/tlpd/maps/download/271/(3)Triathlon.scx

Is it possible in SCM Draft?

SCMDraft -> Sprites -> Unit Sprites -> Place whichever unit you want.

I thought player 12 units showed up in melee? Critters and resources do at least.

Only critters and resources do that. Just tested out. Buildings, special buildings, units, special units, hero units, * like alan turret, addons and nukes doesn't appear in melee game if placed for player 12.

Pretty sure that staredit has no way of placing arbitrary neutral units, only resources and critters. So it makes sense.