The question is too broadbased for a specific answer. In general, you can modify the dropdown list using vba event code or a button to execute code that would copy what you type to a drop down list. But there are conditions that have ot be included in the code to specify where the valid typing will occur and what parameters apply. Otherwise, you might end up copying everything you type to the dropdown list and completely corrupt your project.
As a side note, the forum guidlines for thread titles discourage the type of title you have used. A more appropriate title would have been "Appending drop down lists".