Hm, it's somewhat possible, but there's no menu for that selecting that.
You can get grid cells from cell coordinates (e.g. stored in a Vector3 variable) and place your combatants on them via the event system.
Since the battle start event only works on the whole group at once, you need to use a Custom Event on your combatants and call that from the battle start event to place each individual combatant on their position.
You can set up default custom events for all combatants in Battle System > Battle Settings in the Default Events settings. A custom event is simply a game event that can be started on a combatant via the Custom Combatant Event node and is identified by the event key.
The game event would use a Grid Cell From Coordinate node to find the cell and a Set Grid Cell Combatant node to place the combatant on that cell. You can e.g. use an object variable on the combatant to store the cell's coordinates.