I don't know for sure how Zombieville is creating its textures, my guess is that it is a font, but if you need to, you could probably do something like:
var digitSprites : Texture2D[];
//Fill with 10 values 0-9;
function CalculateScore (score : int) : Texture2D[] {
var scoreImages : Texture2D[] = new Texture2D[3];
//This would create a value to the 100's.
var onesColumn = score % 10;
scores[0] = digitSprites[onesColumn];
var tensColumn = (score % 100) - onesColumn;
tensColumn /= 10;
scores[1] = digitSprites[tensColumn];
var hundredsColumn = score % 1000;
hundredsColumn -= (tensColumn * 10 + onesColumn);
scores[2] = digitSprites[hundredsColumn];
return digitSprites;
//We fill the array in this order so that when we iterate through with a foreach loop the hundreds will be the first value returned.
//You could made a more generic version of this that could keep score up to any number of digits.
}
Then it would just be up to you to format the images into the correct spot.