finalmente ho risolto...

codice:
Public Function fillCombo(q As String, ByRef cmb As ComboBox, field As String)
   Dim rst As ADODB.Recordset
   Set rst = Me.exeQuery(q)
   If Not rst.EOF Then rst.MoveFirst
    Do While Not rst.EOF
        cmb.AddItem (rst.Fields(field))
        rst.MoveNext
    Loop
End Function