Por que no han contemplado una batería auxiliar, del tipo botón, para disminuir los tiempos de encendido, o es posible instalar alguna
It was decided to not provide this functionality in the first version. Thanks for your input, we might consider it for a future release.
High precision GPS needs fresh data to give the maximum accuracy. Using V_BCKP would accelerate the time to first fix, but is not so critical as for normal precision GPS receivers. There is the option to store fresh data in the flash to speed up the next power up by using UBX-UPD-SOS message. You can see more details here: https://www.u-blox.com/sites/default/files/u-blox_ZED-F9P_InterfaceDescription_%28UBX-18010854%29.pdf
Provisions for batteries can make international shipping, or shipping via air very difficult. Even ground shipment within the USA is a concern.
A socket/holder for a CR2032 would likely be an option.