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 (or reactos.dff if you still use rbuild). Example line:
modules\windows\mfc42.dll 1 optional
At the left, you have path to file you want to slipstream. The number is translated to a destination directory (see [Directories] in reactos.dff for the whole list, you can also provide your own). Optional switch enables the build to pass and don't crash if the file is missing.
Now, make a bootcd. The files you have added, will be placed in it, and then installed on 1st stage to chosen destination directory.