We get asked for this a lot and we are looking to address it in the future, for now Michael has a workaround that I have copied below.

1) In Excel:

- Create new file in excel
- File->Properties
- Go to the Custom page
- In the Name edit box type:
  TestType
- In the Value edit type:
  Manual Test
- Click Add
- In the Name edit box type:
  TestID
- In the Value edit type some unique guid (you can use guidgen.exe tool that comes with VS), like:
  D1253494-3D11-4c92-A04E-1A03355D36CC
- Click Add
- Click OK
- File->Save as...
- Save as MHT file

2) In VS

- Right click test project node in solution explorer
- Select Add existing item from context menu
- Browse to your created .MHT file and double click on it
 

3) Alternatively, you can save your excel file as .MHT, open it in notepad and add the following lines:
<!--[if gte mso 9]><xml>
 <o:CustomDocumentProperties>
  <o:TestType dt:dt=3D"string">Manual Test</o:TestType>
  <o:TestID dt:dt=3D"string">D1253494-3D11-4c92-A04E-1A03355D36CC</o:TestID>
 </o:CustomDocumentProperties>
</xml><![endif]-->

 The "Save As" type you would see in "Save As" dialog in Excel is "Single File Web Page (*.mht, *.mhtml)". Sometimes Windows Explorer hides some files, you can configure that by using Tools->Folder Options->View. If you use command line prompt and do "dir" you would see all files.

It seems that "Save As" *.mht is broken in Office 2007 in the sense that it does not save custom properties; if you use Office 2007 you would probably have to manually add these properties. But if you are using Office 2003 everything should be fine.