Yes, it's possible.
The exact part you need depends on how you'll install the drives. If they're going to be permanently fixed in place, i.e. with screws then you can by a basic 2.5" to 3.5" adaptor. I bought one recently to install an SSD drive in a 3.5" bay. It only cost £4 and is made of metal and came with a selection of screws to mount the 2.5".
If you wanting hot-swappable disks, then I think you can try a 'universal' 2.5" to 3.5" SAS caddy like this:
http://www.misco.co.uk/product/173783/StarTech-com-2-5in-SATA-SAS-SSD-HDD-to-3-5in-SATA-Hard-Drive-Converter
I believe Dell also make their own version, so I would assume HP do to.
The above require you to bolt the drive onto the adaptors. But I guess you might have disks already mounted in HP 2.5" caddies ? In which case, you might want to find a 3.5" caddy that accepts 2.5" hot-swap caddies, but if such a thing exists, it'll probably be expensive and only worth it if you're mixing and matching disks all the time.
