Yes you can.
And it's even easier than upgrading the HDD on Sky boxes,mainly because you dont need to mess about with using copy+/and HDD caddies connected to a pc to get around Skys 508gb formatting limit.
With the Humax you can just install the drive and format it in situ.
You need a Sata drive BTW.Make sure you get one thats meant for PVR/AV use,and not just any old drive.
I used a Western Digital WD10 EVDS in mine,if that helps.