Best method to add new items to a databound dropdown menu in asp.net(VB)

I'm looking for some opinions here really. I have an ASP.NET(VB) application with several dropdown menus populated from SQL database tables. Some of them are in datagrids and repeaters. What is the best method to add new items? Currently I have an option to add items on a separate page, but I'd like to have an "Add new" option. Is there a clean method that will add the value to the dropdown and the table without postback? Or if I have to postback, is there a nice looking way to do it in a popup window? Please no links to C# examples. AJAX/Java are ok if they are easy to implement into datagrids/repeaters.