Hi Abin76,
As the other members have implied, we need to know how your computer presently controls the LEDS: linear or pulse width modulated (PWM), before we can provide a design for you to drive additional LEDs from an external power supply.
I guess you don't have an oscilloscope, which would be the best instrument to monitor the output drive from your computer so, instead, could you connect up the simple waveform sniffer shown in the schematic below. The circle with a 'V' in it is a voltmeter which can be a multimeter set to a voltage range above 15V but as close to 15V as possible.
Can you then do the following:
(1) ensure that your computer is switched off.
(2) connect the sniffer to the output terminals of your computer that drives the existing LEDs
(3) turn the computer on.
(4) set the computer to drive the exiting LEDS at a fixed brightness but not full brightness (half brightness would be good).
(5) Note any reading on the Voltmeter
(6) Vary the computer control of the LEDs and note any reading on the voltmeter
spec