Sadly, the technologies are different between the two and you can't combine them.
On the regular +12V RGB header (4-pin), all LEDs of a primary color (R, G, B) are chained together and act simultaneously depending on the input signal. This makes individual LED addressing impossible.
pin #1 - +12V
pin #2 - G (green color)
pin #3 - R (red color)
pin #4 - B (blue color)
On the +5V RGB header (3-pin), there is LED driver IC for each RGB LED package that translates the serial information coming in through the data pin into a specific output for that LED package it is attached to. That method makes single LED addressing possible.
pin #1 - +5V
pin #2 - data
pin #3 - empty (no pin)
pin #4 - ground
Plugging the 3-pin RGB connector to the 4-pin RGB header fries the LEDs since you'd be feeding more than twice the voltage to them (12V vs 5V). And even if the addressable LEDs somehow survive the initial power up, there's no data pin in the 4-pin RGB header to control the LEDs.