I would like to know if it's possible from a script to add IP's in auto-block.
I think you're referring to the blocked sites list. It is possible to add sites and IPs to it via the command line.
See Page 142https://www.watchguard.com/help/docs/fireware/12/en-US/CLI/CLI_Reference_v12_5.pdf
If you chose to write a script to log in via SSH and do this, it could be possible.
I would suggest setting a duration for a blocked site, and not adding it permanently, as if you're adding entries to this list via some automated script, it can grow very large quickly. Each firebox model has a limit as to how big the list can be.