You can automate Excel, write your value to the worksheet, then update the object frame.
Private Sub cmdInsert_Click()
Dim strPath As String
Dim oExcel As Object
Dim oSheet As Object
Set oExcel = CreateObject("Excel.Application")
oExcel.Visible = True
strPath = Me.ExcelFrame.SourceDoc
'Debug.Print strPath '
oExcel.Workbooks.Open strPath
Set oSheet = oExcel.ActiveSheet
'Debug.Print oSheet.Name '
oSheet.Range("A1").Value = Me.txtA1
oExcel.ActiveWorkbook.Save
oExcel.Quit
Set oSheet = Nothing
Set oExcel = Nothing
'acOLEUpdate action requires Enabled = True '
'and Locked = False '
Me.ExcelFrame.Enabled = True
Me.ExcelFrame.Locked = False
Me.ExcelFrame.Action = acOLEUpdate
Me.txtA1.SetFocus
Me.ExcelFrame.Enabled = False
Me.ExcelFrame.Locked = True
End Sub
Edit: The example was based on an external workbook file which is linked as the source for the form's object frame.
To link a worksheet, choose the "Create from File" radio button, check the "Link" check box, and browse to select the workbook. That's the way I did it with Access 2007. As I recall, it was similar with Access 2003.