The signature is inserted automatically. However it looks like that only happens AFTER the message is displayed. So you would need to re-order your code a bit. First, display the message; then get its contents (which will include the signature); then add your text and put the contents back. You'll get some flickering but it shouldn't be too bad, hopefully.
Code:
objMail.Subject = "Here is the reportr"
objMail.Display
strMsg = objMail.HTMLBody
strMsg = Replace$(strMsg, "<BODY>", "<BODY><p>New text</p>")
objMail.HTMLBody = strMsg
(VBA code - that's all I have at work, sorry)