Dynamically create WIX files without having to edit the wix files manually
For WiX v3 (and a free solution), look at heat.exe. If you have more advanced harvesting needs, there is a commercial option from FireGiant**.
** Disclaimer: I am the benevolent dictator of the WiX Toolset and CEO of FireGiant. We contribute to the open source project and provide commercial solutions around the project. Pick what works best for you.
For Wix 2 there was a tool called Mallow that did this. It was freeware and I got the source somewhere. This functionality may also be available in Heat.exe at this point, but I am not sure.
It also seems some guys in the Wix community are experimenting with a sort of automatic scanner feature where you can put files in a pre-defined release location and new msi files are continually created - ala "continuous integration".
Are you looking to do this for a lot of packages, or just for one or a few ones? Let me know the scope and I can send you the Mallow code, or dig up links for the "continuous integration" oriented solutions.