Full documentation is available at https://www.qubes-os.org/doc/usb-qubes/. You should read the Security Warning about USB Input Devices before doing this.

To enable a USB keyboard for dom0 (making it available to all VMs) modify /etc/qubes-rpc/policy/qubes.InputKeyboard on dom0 to reflect:
#sys-usb dom0 allow,user=root sys-usb dom0 ask,default_target=dom0 $anyvm $anyvm deny

You will then be prompted to grant USB keyboards access any time they are connected. To also allow USB keyboards to enter the LUKS and login passwords please refer to the official documentation.