It is quite difficult to say without the Batch file code (because it is a .bat isn't it?).
However, just in case you don't know, the SUBST command makes a drive out of a directory under MS-DOS. DosBox built in DOS does not support it, but if you wish you may get Freedos or a similiar DOS operating system and install it into DosBox.
Anyway, my advice is that you edit the batch file, get rid of the SUBST command, create a virtual CD-ROM drive via the MOUNT command and copy the contents of the directory referred byt the subst command there, making sure that you choose the same drive letter specified by SUBST, of course. Of course, this is pure speculation without seeing the batch file itself.