Karadjgne, let me explain my thinking. Unfortunately we do not have the real identity of the fans, nor any info on their wiring, so we are guessing. OP claims there are LED's in the fans that are turned on and off by a case front switch. I queried whether those LED's really are in the fans' frames like normal LED fans, or actually are only separate LED units mounted in the case independently of the fans. No answer on that, either.
However, assuming OP's statements are correct, there must be some separate wiring for those LED units. That is not common in LED fans, but you have cited a design that DOES do just that. So MAYBE something like that is in OP's system.
In most LED fans the LED's simply are connected in parallel with the motor supply voltage, whatever that is. In a 3-pin Voltage Control type of fan, this means that the voltage to the fan (and LED's) will range from 12 to 5 VDC, and we know that such fans' lighting does get dimmer at slow fan speeds. Now, almost all such LED's really are designed to work with 5 VDC. My guess is that the LED's actually are fed though a small dropping resistor to prevent overvoltage to them. But of course no maker will discuss that level of detail with buyers who do not need to know this and might just get confused. Now in this particular situation, OP has some kind of switch and case wiring that uses both 12VDC and 5 VDC supplies to the front panel switch, and also appears to have separate wiring to the LED's. As I said, I believe the case itself has a few LED's (like switch ring bezels) independent of the fans, and these MAY be fed only 5 VDC. So IF that is done, it also would be possible to wire a separate 5 VDC supply to any LED's in the fan frames IF they are designed to work on that voltage supply without any dropping resistor. But all that really is speculation, because we do not have enough details.
Bottom line certainly is, as several have said, any common design of LED fan added to the system and plugged into a mobo fan header can NOT have its LED components controlled in the same way unless OP is prepared to do a bunch of custom wiring and detective work. Unlikely.