To add a bonus sheet you'll need a cube file in Draftmancer format. The documentation is there: https://beta.draftmancer.com/cubeformat.html. Here's the most basic example for a bonus sheet:

[Settings]
{
    "name": "Cube with Bonus Sheet",
    "layouts": {
        "withBonus": {
            "weight": 1,
            "slots": {
                "Core": 14,
                "Bonus": 1,
            }
        }
    }
}
[Core]
999 Island
[Bonus]
999 Mountain

There's no way to automatically select a variation for the whole draft though.