Adding more values by choose in the list

Question

Hi all!
Normally with TextBox control, when I choose an item from the list, the Text property value assume the value of the item choosed.
How can I modify this behaviour by adding the value of the item choosed to the existing value (if exist)? 
So I would choose many items and see they in the TextBox.
Can I do this?

Thanks and sorry for my english!
Enrico.