yes, you can.
but you probably don't need to completely customize it. why don't you see if you can manage by assigning textformats for the standard combobox:
cb.textField.setStyle("embedFonts", true);
cb.textField.setStyle("textFormat", tfor1);
cb.dropdown.setRendererStyle("embedFonts", true);
cb.dropdown.setRendererStyle("textFormat", tfor2);
i tried
var tform:TextFormat = new TextFormat();
com_txt.dropdown.setRendererStyle("textFormat", tform);
tform.size=8;
//com_txt.dropdown.setPosition
var tformm:TextFormat = new TextFormat();
tformm.size=16;
com_txt.textField.setStyle("textFormat", tformm);
com_txt.dropdownWidth = 40;
com_txt.dropdown.rowHeight = 10;
North America
Europe, Middle East and Africa
Asia Pacific