In order to place any files on ReactOS Bootcd and having them installed automatically, you are required to checkout and be able to build ReactOS source code.
Next step is placing files in source code repository correctly. By definition, you should use modules\windows directory, though you could use any path inside ReactOS code repository.
After placing your files, please edit \boot\bootdata\packages\reactos.dff.in. Example line:
modules\windows\mfc42.dll 1 optional
At the left, you have the path to the file you want to slipstream. The number is translated to a destination directory (see [Directories] in reactos.dff.in for the whole list, you can also provide your own). The optional setting instructs the build to ignore the file if it is missing (instead of aborting).
Now make a bootcd. The files you have added will be placed in it, and then installed to the chosen destination directory during 1st stage setup.