Zyxel NBG6817 external overlay rootfs on /dev/mmcblk0p10 with OpenWRT 21.02.3?

I have Zyxel NBG6817 and flashed openwrt-21.02.3-ipq806x-generic-zyxel_nbg6817-squashfs-factory.bin via stock firmware.
All was flashed ok except there is small size rootfs and there is quite big and free /dev/mmcblk0p10 for about 3 Gb.
Is it possible to engaged it for external overlay ?
Are there all ncesessary pactehs built in to OWRT 21.02.3 ?