Private Sub Text1_KeyDown(KeyCode As Integer, _
                            Shift As Integer)
    If KeyCode = 46 Then
       KeyCode = 80
    End If
    
End Sub

Private letra As String

Private Sub Text1_KeyDown(KeyCode As Integer, Shift As Integer)
  letra = ""
  If KeyCode = 46 Then
    letra = "p"
    KeyCode = 0 'así no tiene que borrar el carácter
  End If
End Sub

Private Sub Text1_KeyUp(KeyCode As Integer, Shift As Integer)
  If Len(letra) > 0 Then
    Text1.Text = Left(Text1, Text1.SelStart) _
                 + "p" _
                 + Right(Text1.Text, Len(Text1) - Text1.SelStart - 1)
  End If
End Sub

If KeyCode = vbKeyDelete Then 
KeyCode = 0 ' Anular el delete
SendKeys "{p}" ' Simular el tecleo de la p 
End if