In a large mixing bowl, break up the destroyed cake more than it already is!
Add the alcohol while mixing to form a "dough" like consistency that is firm and scoop-able but not super liquid and falling apart. You may have to adjust the amount of alcohol you are adding to get a good consistency. Traditional rum balls are not wet, but dense like a fudge brownie
Scoop them into balls and then roll the balls to form a nicer round shape.