adding a file into existing msi file

Hi,

I searched all over google groups to find a solution this problem but
could not find answer, though some vague suggesiions I found.

Here is the problem.

I have built a .msi using the Visual Studio.NET (actually the setup
project was created automatically for me...!!). I need to add a new
file into this msi file outside of the visual studio.NET. Inside the
overridden installer file of msi I determine to use the file or not
depending on whether its been added to msi file or not.

The use case is: for the customer to add a file later into the msi
file for some functionality only if they want to use it. They
definitely will not have VS.NET.

I tried to use MaSaI editor, and use its "add file functionality but
once i added the file the MSI stopped working giving errors.
Can I do it using MaSaI editor? if yes how?

I am trying to add a binary file (.mpp file to be precise). Is it
possible to do this? If you can suggest any links related to this,
thats also fine.

thanks a lot in advance.