Add custom combo box (aka select/drop down box) to toolbar

Hello,

I've been looking for a way to put a custom combo box in the toolbar of the CKEditor but haven't found anything so far.
I know how to add custom buttons as a plugin like this:

editor.addCommand(b,a);
editor.ui.addButton("button",{});

Is there a similiar way to add a combo box similar to the font chooser drop down?
Any help is welcome