This is probably going to be a long shot... so it may or may NOT work.
The way I see it is... if the multiplier card needs any type of drivers, these would NOT be installed on the firmware of the DNR-322L so the use of it will failed. If the multiplier card does NOT need any other drivers, then you may use it, but at this point you may have a problem with the DNR allocating all other drives since as far as the unit is concern it may only have room for allocating two volumes. It is really a trial an error... and obviously this would NOT be supported by D-Link if anything goes wrong with the recording and normal operation.
Not sure if any one here may or may not able to help... but that's my two cents.