is it possible to install a saddle with 7x9 mm rails?

Without modification to the seat post, I think you would risk damaging the carbon fiber.

how can I will update this?

@Dave you have the seat post for saddle 7x9 mm for Garmin bike plus?

No, I do not.

Does that thing exist ? I would like to have one as I have 2 saddles with 7x9 rails