If by wav files you mean MS Windows .WAV files, then no, I dont think that can easily be done. But it is possible to load new sounds that have their own samples (rather than simply being new sets of parameters building new sounds from the existing samples, which is all you can do from the front panel Custom Voice Edit section). It takes some external software to do this on your own, but you can buy sample disks from a library EMS made for Yamaha for their earlier model, the PSR-2600. Except for RAM limitations (the 7000 has less RAM and thus cant load some of the larger sounds such as Gospel Choir), any sound for the PSR-2600 can be loaded into a PSR-7000. The disk formats are compatible, too.
As for hard disks, it is possible, but Ive yet to come across the required cable. The hard drives themselves should be a 2½" EIDE hard drive, such as are designed for use in laptops and notebooks (note: the PSR-7000 will only format the first 350MB of the drive for its use, so installing a larger drive would be a waste of capacity). The drive mounts in brackets (already installed) located on the bottom of the PSR-7000s floppy drive.
There is a pair of connectors present on the digital board where you would plug in the EIDE card and a power cable for the drive. Problem is, theyre non-standard. You see, since these drives are meant to be installed into laptops/notebooks, and since there are such tight space constraints in such devices, most dont even bother with cables at all: the drives plug directly into a connector sticking up at a 90° angle from the laptops motherboard, just like the PCMCIA connectors do. Even where cables exist, theyre only an inch or two long not nearly long enough for our purposes.
2½" EIDE hard drive connectors are a condensed version of the EIDE connectors on a 3½" desktop/tower-sized hard drive. The pins are smaller and closer together, and there are four more of them, separated slightly from the others, which bring power to the drive (rather than using that fat D-shaped four-fat-pinned cable that people who assemble PCs so know and loathe, or even the smaller version used for 3½" floppy drives). Since the PSR-7000 motherboard only has two pins for drive power, I dont know how that would translate to the power requirements necessary for the laptop drive.
I have a PSR-7000 and an IBM 351MB Model DBO[0?]A-2351. If anyone knows where I could get a cable for it that would be long enough (18" or so), and have the right connectors (the combined connector for the drive end, the separate EIDE/power connectors, in mini size, for the PSR-7000s digital board), please post here! Ive searched the Internet, and even called custom cable-building companies (Belkin, etc.), to no avail. :-(