Christine,

Copy the code below, right-click the sheet tab, select "View Code" and paste the code into the
window that appears.

Change the
Set rngPCode = Range("D:D")
to the column where the postal code will be entered.

This event  will allow either 6 character entry or 7 character entry (with the 4th being a space) in
the column.

HTH,
Bernie
MS Excel MVP

Private Sub Worksheet_Change(ByVal Target As Range)
Dim rngPCode As Range
Set rngPCode = Range("D:D")
If Intersect(rngPCode, Target) Is Nothing Then Exit Sub
If Target.Cells.Count > 1 Then Exit Sub
On Error GoTo Reset
Application.EnableEvents = False
If (Mid(Target.Value, 4, 1) = " ") And (Len(Target.Value) = 7) Then
  Target.Value = UCase(Target.Value)
Else
  If Len(Target.Value) = 6 Then
     Target.Value = Left(UCase(Target.Value), 3) & " " & Right(UCase(Target.Value), 3)
  Else
     MsgBox "You need to enter a valid Postal Code"
     Application.Undo
  End If
End If
Reset:
Application.EnableEvents = True

End Sub

Excel lets you make formats for numbers but not masks for text
Only thing I can suggest is using formula =UPPER(LEFT(A1,3)&" "&RIGHT(A1,3))
to do the conversion
Later you could use Copy followed by Paste Special Values to convert that
column to text. This allows you to delete the input column
best wishes