Can worn out balls in THK bearing block be replaced ?
Yes they can. It's not that difficult to do on trucks. It is more difficult on ball screws, but still doable with some.
Remove one end cap and measure several balls until you are sure you have the dimension. Then order some. Make sure you count the balls on each side. This will work fine as long as the rail isn't worn. I think the rails in THK are harder than the balls, but not certain. I do have a small set where the balls are football shaped, but the rails appear fine.