Theoretically, it's possible, but it's honestly not the best idea.
The entire Seasonic S12D and M12D series power supplies, as well as the units made from them, (including your HX620) don't have their rail specs properly labeled. For example most S12D's are advertised as dual rails, but are in fact single rails. (which are typically better at those wattages, they just advertise them as lesser units for some reason) The rail layout alone is pretty darn confusing. Check out this
review.
If you're adventurous and brave, grab a multimeter and find out which pins are 12V, 5V, Ground, etc. Then go ahead and mod at your own risk. The 5 pin design probably has redundant ground wires. (no clue what else it could be used for, since it only needs 4 pins)