At the moment you could only change that using an Hex Editor like HxD. To do so open your save file, go to offset 0x7E844 and copy the 18 (0x12) bytes from 0x7E844 - 0x7E855 over to offset 0x7E85C - 0x7E86D.
After that you need to fix the checksum. Open your save in BW Tool again, open the Memory Link Editor and click Save and Exit. (The Update Checksums button in the main window doesn't take the DLC region into account.)
@suloku In ML_Block2 TID (0x5C), SID (0x5E) and Name (0x60) needs to be set as well.