Can I use the Exchange Default template in Exchange Web Form.

I create a new Exchange Web Form to handle the message's Open request.

oRec.Fields("DAV:contentclass") =
"urn:schemas-microsoft-com:office:forms#registration"
oRec.Fields("urn:schemas-microsoft-com:office:forms#contentclass") =
"urn:content-classes:message"
oRec.Fields("urn:schemas-microsoft-com:office:forms#cmd") = "open"
oRec.Fields("urn:schemas-microsoft-com:office:forms#executeurl") =
"CustomMessage.asp"

Exchange will use my form to render the email when user open it in owa.

I can write some script and some HTML in CustomMessage.asp, it works
well.

Now what I want to do is: In this Form, I want to use  the Exchange
default "Open" template, nor I write a new one.
Maybe my description is not very clear, I have a example to descript
it.
I have some emails, some MessageClass are IPM.Note.aaa, some are
IPM.Note.bbb.  when user visit them in owa, I want to do the logic:
if it's MessageClass is IPM.Note.aaa, I want to make this mail's
subject as "aaa", if is IPM.Note.bbb, make it's subject as "bbb".
modify their body via some logic. after that, let exchange render this
email use it's default template.  so that the user will no detect the
email has been modified

Thank you!.