How do you fill a garrison with creatures, and how do you combine creatures into a unit?

I need to populate the map with creatures for the garrison and combine them into a unit for compactness.

P.S. Since I'm writing again, how do I make or remove impassable terrain?

Added after 1 hour 38 minutes
And now the editor is acting up; saved maps are not displayed in the game and give an error, also when reloading the map.