There are direct HD style banks just like user style banks. You can either load and save the styles from floppy drive into direct HD bank or copy styles directly into the banks using USB from your computer.

The user styles can be replaced using the load feature under disk menu. These can be loaded either from HD or FD.

Yes, you can add styles to the empty spots of the base styles. You can also overwrite all the existing base styles, which is one of the best feature that Korg arrangers have.