A relativlely common problem this, transferring data to/from MS-DOS disks to the Dragon. First a couple of points: the disk chip inside the Dragon (WD2797) will automatically read PC sectors under DragonDOS just by using SREAD (this is fully explained in the DOS I/O set of articles). However, you will only 'see' the first half (256 bytes) of the 512 byte sector returned in the strings as DragonDOS is only expecting a 256 byte sector. The full sector will be held in a buffer pointed to by the locations $EE:$EF directly after the transfer. However, this is rather a dangerous method as DragonDOS will not allocate space for sectors of this side and you are liable to corruption if you do this. It is therefore better to use the read/write logical sector machine code routines and pointing them at your own 512 byte buffers.
So at a very low level, you can dump your DOS file onto sequential DOS sectors on the PC and read it back on the Dragon.
There are a couple of PC disk transfer programs for OS9 here which will read/write a number of PC disks from 5.25" SS/DS up to 3.5" DS/DD, and the old Compusense PC_Convert program which runs on PC and will read DragonDOS single sided disks only. For DragonDOS, there is the ReadPC program by Martin Vermeer which I did some tinkering around with a few years ago. Mail for details.