In our grid\samples\celltypes\comboboxes I added a gridControl1.CurrentCellValidating event in Main.cs, and also made StringCollections items a member of the form (and not a local variable). With those changes, adding a new item to teh list in CurrentCellValidating seems to work for me. The item is added as you leave the cell of the of the editable combobox using a choice list.
private void gridControl1_CurrentCellValidating(object sender, CancelEventArgs e)
{
String s = this.gridControl1.CurrentCell.Renderer.ControlText;
if(items.IndexOf(s) == -1)
{
this.items.Add(s);
Console.WriteLine("New Item Added");
}
}