You can use the CurrentCellShowingDropDown event to set values into a droplist. If you have a list of strings you want to use as the drop values, then you can use a StringCollection object and set it as the ChoiceList property of the cell style.
Here is a sample that changes the ChoiceList shown in column 2 based on the value set in column 1 using CurrentCellShowingDropDown. You can see other choicelist usage in the Grid\Samples\CellTypes\ComboBoxes sample. There is also a KB on dynamic combobox lists though it uses DataTables and not choicelists. http://www.syncfusion.com/Support/article.aspx?id=567