jeffparker,
There ae specific 5.25" to 3.5" (and 2.5") drive bay adapters. I would be very surprised if HP does not make such an item.
There are a variety of third-party designs designs and a couple can be configured for both 3.5" and 2.5". Here's a page of examples in the US:
I've thought about one of these for a long time for one of our z420's, but my thought is to use a hot swap version for a backup drive, so all the systems in the office share one backup that can be kept off site. An example:
One other possibility if you need more capacity, is to have a PCIe or RAID card with eSATA and you can connect to an NAS stack / enclosure-2,3,or 4 drives. For example:
http://www.newegg.com/Product/Product.aspx?Item=N82E16816129101
Cheers,
BambiBoomZ