ya you can make a external power supply ..
voltage regulator adjustable with a volt meter :>
but do you have a pot that can handle up to 2W for the current regulator?
if not then you will have to set it at .. wathever mA
uhh yes..
and you use resistors to set the current... doesnt matter what regulator it is..
so if you want to use it for a phr diode
use 10+1 ohm resistors in series for 11ohm so you get about 114mA..