Well without having to reinstall, that is quite impossible unless you have a spare drive to move your install onto. What you want is a RAID setup. There are two types of RAIDs that you would probably use. Either JBOD, which just combines the drives into a single partition, or a RAID 0 which will split the files across both HDDs, but if one drive fails, you lose everything on RAID 0, unlike JBOD.