I'm using Vdd=1.9V for 0.18u technology and obtaining the PTAT and CTAT currents. Now i've to obtain the current reference output using these currents. How is it possible to do so?
Not at all - if they are really "good" current sources: high output impedance against the input impedance of the summing node, e.g. "diode" (gate-to-source) connected "master" transistor of a current mirror - the "slave" transistor mirrors the added current.
Didn't you study the circuit which you copied? Not only subtracting, also adding currents is shown there!