I found an alternative way, and I would like to share it.
Maybe there's somebody who will like my idea.
I left this code in retroarch.cfg, so it's active for all the libretro cores:
input_turbo_period = 6
input_turbo_duty_cycle = 3
and using the RGUI I assigned the enable Turbo function to the same button I want to apply the Turbo function.
As I'm FBA roms, most of the game doesn't have the same controller mapping, but with this method I can have a turbo setting per rom. Unfortunately I don 't know how to save this settings for each single rom and not for the entire core.
so I started to place different config files for each rom I want to made this Turbo setting permanent, adding a file with the same name of the rom in the "Roms" folder.
Exemple: if the rom name is "mslug.zip" and I want to set Turbo on the "Fire" button (which in my case is button 1), I created a file mslug.zip.cfg with the code :
input_player1_turbo_btn = 1
input_player2_turbo_btn = 1
Hope this can be useful.