The word add is misused here and if I get what you want to do, you will need some sort of electronics. You could sum with a resistor and the voltage would would correspond to the sum of the currents. You would then need to translate that voltage to a 4-20 ma signal. If you used a 125 ohm resistor, you would get 1-5 V then you could use a signal conditioner to convert. That is _if_ the current sources behave in that connection.
Actually, on reflection there is a cheap simple way if the input you are driving is the typical 250 ohms across a voltage input, Simply place another 250 ohm resistor across it making it a 8 to 40 ma input.