Dim DB As DAO.Database, _
    Tabl As DAO.TableDef, _
    Fld As DAO.Field, _
    Idx As DAO.Index, _
    B As Boolean
Set DB = DBEngine.OpenDatabase("имя БД", , False)
Set Tabl = DB.TableDefs("имя таблицы")
For Each Idx In Tabl.Indexes
 For Each Fld In Idx.Fields
  If Fld.Name = "имя поля" Then
   B = True
   Exit For
  End If
  If B Then
   Exit For
  End If
 Next
Next
If Not B Then
 Set Idx = Tabl.CreateIndex("имя индекса")
 Idx.Fields.Append Idx.CreateField("имя поля")
 Tabl.Indexes.Append Idx
End If