connect your voltage ref in parallel with the pot. Connect the -ve end of the ref/pot to the anode of a diode(1n914 etc). Connect the cathode of the diode to ground. Bias the pot/ref with a resistor to V+. Connect the wiper to the base of an npn transistor. Connect the emitter to Re (an emitter resitor) connect the other side of Re to ground. The collector of the npn is your current source, the single diode provides some Vbe compensation.