so, you've wanted the white hawk to appeared in your map?
I think there's no way to do it, it's hard coded
and for your next question, spawn 1 random unit from all tilesets..
I see it can't be done, it just spawn the creep from your current tilesets
And...
I have another great and need much effort, it is using trigger to store all data of the unit and spawn from the trigger
Like this
Variable: ran_creep = unit - type (array)
Set ran_creep[0] = furbolg
Set ran_creep[1] = furbolg tracker
Set ran_creep[2] = dark troll shadow priest
Set ran_creep[3] = dark troll warlord
Set ran_creep[4] = forest troll warlord
And so on, then using trigger like this:
Action
Unit-create 1 ran_creep[random number between 0 to 4] in blah blah