Can I use the following power supply as a constant fixed current source?
It says in the spec you linked that the current adjuster only works on the 0-30V output and can limit from 10mA to 3.2A. I would think that for feeding a current into a resistor this will be OK but don't expect miracle performance, for instance if set to 20mA any resistance higher than 1500 ohm will cause the output voltage to want to go greater than 30V and of course this cannot happen. Also, if you have voltage limit set to 15V, then don't expect the current limit to stay limiting/controlling when the external resistance is above 750 ohms.

EDIT If you are not satisfied with the performance of the power supply and you wanted to try a different method consider this technique: -



With R1 at 312.5 ohms, output current should be 4 mA plus a little bit from the adjust pin (maybe 50 uA). For 20 mA, R1 needs to be 62.5 ohms