Hi, thanks for posting!
By definition, the declarations in the config XML are internal to the software - they aren't documented or published as they can potentially change in future and aren't really intended to be changed manually. Where possible, you should use the options in the configuration window. With that said, there is nothing to stop you from modifying these files manually if you have a good reason to do so.
At time of writing this (v2.10), the AdditionalFilesToInclude values are semicolon delimited and must be relative to the project file (or solution file when applied at solution level). For example:
<AdditionalFilesToInclude>WiX\*.*;..\References\**.*;..\CrossProject\**.*</AdditionalFilesToInclude>
Cheers,
Remco