PT1000 2 wire probe. Can I add third wire?
Sure you can add the extra wire. Just bring it out to the element. 

It should be noted that this is not a proper 3-wire RTD scheme and the extra wire only halves the error. A proper 3-wire scheme will reduce the error (in theory) to zero, or at least to the mismatch in resistance between wires. 

Here is an example of such a scheme: 







simulate this circuit &ndash; Schematic created using CircuitLab

To see how this works, consider equal resistances RL in each line. The voltage drop from ground to the lower end of R1 is \$(Ix-2\cdot I_X) \cdot R_L\$. The voltage drop from the top of the element to Vsense is \$+I_X \cdot R_L\$. The voltage drop across the element is just \$I_X R1\$. 

So the voltage at Vsense relative to ground is: 

\$-I_X \cdot R_L +I_X \cdot R_L + I_X R1 = I_X R1 \$

And the line resistances (if matched) don't have any effect on the measurement.