I'd be tempted to use SATA power splitters.
Hard drives are only a few watts each, even under load. If you want to add only two more, you'll be increasing your load on the PSU by a whopping 12W or so, and Dell would never leave that little headroom in their PSU. If your PSU assplodes because of an extra 12W, then it was way past due for replacement anyway!
The steady-state draw isn't the concern. The most likely time for problems to occur is when the system is initially powered up and all of the drive spindles try to start at once. Spin-up power draw of a hard drive can easily be several times what the drive uses in normal operation.
Given that it is a file server, power-cycling the system should be a relatively rare event though.
If the power turns out to be a problem, another potentnal option would be an eSATA card and a couple of external eSATA drives...