Okay, I think I see the problem. If you just replace the trimmer with your external potentiometer, you could blow out your LEDs by turning the current up too high. So, you need a resistor in series with the potentiometer in order to limit the current to the max the LED can handle. So this is complicated.
I am going to assume that the current rises with increased voltage at the slider.
You need to set the trimmer to the max current your LED will be using then read the voltages on the 3 terminals of the trimmer. You should get something like 0V, 5V and 3.6V (for example) on the slider terminal. That tells you voltage you need for the max current.
Now you calculate the resistor needed to limit the voltage to that voltage (3.6V in the example above) when the potentiometer is turned all the way up. You do this by making a voltage divider with the resistance of the potentiometer (say 10K) and the series resistor. If the series resistor is Rs, and the resistance of the potentiometer is Rp and the voltage across the potentiometer is V, and the desired max voltage is Vmax. You can solve for Rs:
Vmax / V = Rp / (Rs + Rp)
(Rs + Rp) * Vmax / V = Rp
Rs + Rp = Rp * V / Vmax
Rs = Rp * (V / Vmax) - Rp
In the example above:
Rp = 10000
V = 5
Vmax = 3.6
Rs = 10000 * 5 / 3.6 - 10000
Rs = 3889
So you would put a 3889Ω resistor between the terminal of the pot connected to the higher voltage. More resistance is OK since it would limit the current to a lower value.
Hope this helps.
Bob
