This question would have been better posted in the Developer Forum.

For custom features, ControlType can be TextBox, CheckBox or ISettingControl. For ISettingControl you implement a UserControl.ascx that implements the ISettingControl interface. You also have to put in the path for the UserControl.

So to make a dropdown you would have to implement a UserControl that implements ISettingControl.

If you are working with the mojoPortal source code, you can see an example that implements a dropdown list as ISettingControl in the Web/Controls/GMapTypeSetting.ascx and GMapTypeSetting.ascx.cs

This dropdown can be seen in the settings for a Google Map instance.

Hope it helps,

Joe