A combo box does have an AddItem method. It was new in, if I remember
correctly, Access 2002, but it was added to the combo box in the same
version in which it was added to the list box, so if you're using a version
of Access in which list boxes have an AddItem method, then you're using a
version in which combo boxes have it, too. Unless you're using some ActiveX
control instead of the intrinsic Access controls?

You can also assign a recordset directly to the RecordSet property. Here's
examples of both methods ...

Option Compare Database
Option Explicit

Dim mrst As ADODB.Recordset

Private Sub Form_Open(Cancel As Integer)

   Dim cnn As ADODB.Connection

   Set cnn = New ADODB.Connection
   With cnn
       .ConnectionString = "Provider=SQLOLEDB.1;" & _
       "Integrated Security=SSPI;" & _
       "Persist Security Info=False;" & _
       "Initial Catalog=Northwind;" & _
       "Data Source=(local)"
       .Open
   End With

   Set mrst = New ADODB.Recordset
   With mrst
       .ActiveConnection = cnn
       .CursorLocation = adUseClient
       .CursorType = adOpenStatic
       .Open "SELECT CategoryID, CategoryName FROM Categories"
       .ActiveConnection = Nothing
   End With

   Set Me.Combo0.Recordset = mrst

   Me.Combo2.RowSourceType = "Value List"
   Do Until mrst.EOF
       Me.Combo2.AddItem mrst.Fields("CategoryID") & _
           ";" & mrst.Fields("CategoryName")
       mrst.MoveNext
   Loop

End Sub

About my previous comment " List Box has AddItem, but not Combo Box."
I put it after reading some articles on the web without checking if it
actually has it