Adding items to RibbonDropDown at runtime

Try the following directly inside the Ribbon Class:

RibbonDropDownItem item = this.Factory.CreateRibbonDropDownItem();
item.Label = "Text";
combo.Items.Add(item);

Try this. This should work for you.

RibbonDropDownItem item 
      = Globals.Factory.GetRibbonFactory().CreateRibbonDropDownItem();
item.Label = "First Name";
this.cbRecent.Items.Add(item);