Can I add a hard drive?

Hello all,

I've been mulling over a purchase of a 27" iMac, but I want to maximize SSD space. Unfortunately this comes at huge cost, so I'd rather leave out a HDD. I figure I can always find a good one on newegg or something on sale, so why pay Apple, right?

Anyways, I know the reverse of this question is true. One can add an SSD to a HDD-Only iMac, but can I add a HDD to an SSD-only iMac? Has anyone attempted this? I'll assume the SATA ports are open, and there won't be any data/power cables. Whats the right length for the cables? Is there a proprietary caddy? Mounting screws that aren't provided?

Any information would help. Thanks!