In order to write to the html body, you need to use the Outlook Word Editor Inspector and then you can treat the body as if it was a Word document e.g.
Code:
With olEmail
.BodyFormat = olFormatHTML
Set olInsp = .GetInspector
Set wdDoc = olInsp.WordEditor
Set oRng = wdDoc.Range
oRng.Collapse 1
This entry point at the start or the message body. As long as you know how to manipulate strings you can add whatever you want to that body.
Or you could simply use
http://www.gmayor.com/ManyToOne.htm in one to one mode and merge directly to the message body