You can already add another entry to the subject list, simply by clicking on the content control, then choosing Developer>Controls>Properties>Add. Having to do it that way ensures greater consistency in the data entry. Otherwise:
Code:
Sub Modify()
Dim CCtrl As ContentControl
For Each CCtrl In ActiveDocument.ContentControls
With CCtrl
If .Type = wdContentControlDropdownList Then .Type = wdContentControlComboBox
End With
Next
End Sub
You will also need to edit the existing VBA code to change the wdContentControlDropdownList references to wdContentControlComboBox.