Please read the instructions in the official RockBox for FUZE manual.
Use the rbutil to generate
Then open the file
.rockbox/langs/english.voice (or whatever language you just created) up in a hex editor and change the
Byte at position 0x07 to
0x35 = 53d, the target ID of the Fuze. Now it should be recognized.
You can find all target IDs in the rbutil.ini.