How does one make units that come with other units

[Sombre]

To your more general question, the answer is no, because the vanilla game units weren't made with mod commands, they're hard coded.

There is no mod command to give that ability to a unit.

The only thing you can do is #copystats a unit which has the ability. You can then alter, say, imps, so that they are an entirely different unit and voila you have a unit that summons whatever you want when it enters combat. Unfortunately you've also changed what imps are, which causes problems whenever a spell/unit/event creates imps, since they'll create whatever you changed them into instead.