When you add new trainers to fight against, there's usually no text associated with them while fighting.
The solution is found on narc a/0/5/7.
The file itself, without the header, is 6868 bytes long.
Each reference is 4 bytes long.
6868 / 4 = 1717 references.
Each of these references points to its corresponding message on text file 728 from narc a/0/2/7.
To add a new message on text file 728, use Spiky's DS Map Editor.
Format is TT TT MM PP
TT TT is the Trainer slot number (little endia