here is some code that I got from somewhere, not exactly what you are after but it may point you in the right direction.
>>>>
You must use the DAO Properties collection of the Field object. A property such as Format does not exist a priori, if it has never been set before, you must first create it, and append it to the Properties collection. You can use the following function for this purpose:
Sub SetFieldProperty(fld As DAO.Field, strName As String, intType As DAO.DataTypeEnum, varValue As Variant)
On Error GoTo ErrHandler
fld.Properties(strName) = varValue
Exit Sub
ErrHandler:
If Err = 3270 Then
' Property does not exist
fld.Properties.Append fld.CreateProperty(strName, intType, varValue)
Resume Next
Else
MsgBox Err.Description, vbExclamation
End If
End Sub
Apply it like this in your code (in the For Each tfld In tdf.Fields loop):
SetFieldProperty tfld, "Format", dbText, ">"
SetFieldProperty tfld, "InputMask", dbText, ">CCCC;0;_"