Is there a way to control LED strips through the 4Pin RGB header from the motherboard and power them by a separate power supply, because my motherboard can only supply strips of up to 3m and my PC power supply has not enough Watts left to supply 20m of LED strips?
Trying to use or otherwise connect in a separate power (12 volt) supply could cause some sort of electrical loop or short circuit that would not end well for the hosting computer and/or its' components..
And you would need to have a very good understanding of the necessary pinouts to modify power to and control of the LED strips.
What make, model, wattage, age, and condition is the current PSU?
My recommendation would be to install a new PSU with wattage capable of supporting the PC and LED strips.