It is basically dad, it's possible from fastboot mode to activate de usb debug mode, and if it is, how can i?
I was just commenting on another similar question. Is your phone rooted? You can try this post's solutions to see if one of the methods work.
I believe you can flash a custom recovery via fastboot in case it's not present. Eg. -
fastboot flash recovery <<recovery-name.img>>
Then use volume keys to select recovery, power button to enter.