Install in an external disc


I bought a Raspberry Pi 3B+ and an expansion kit X830 to attach a SATA hard drive to it as an USB drive.
I downloaded the img file and dd it to the SD card.

There is any way to customize the install process before running the Raspberry for first time?

There is any way to install the OSMC using the SD card as boot partition, and USB hard drive as root partition?

Yes, you have that option when you use the OSMC installer and choose USB Install as target.