How to insert an item to a List box / Radio list using event script

Is it possible to add/insert an item to a list box or radio list using event script. So far I am only able  to add item to list box via the getmemberlist from a data source or using the insert item under the properties of list box. I am trying to populate a list box on run time, allowing user to add item to the list box on the fly using for example input list.

Any help would be appreciated