Extending the wires of soil moisture sensor without voltage drop
Although you did not identify the sensor or what kind of wire you are considering, it seems quite unlikely that you will get any voltage drop unless you use very tiny wire.  

Assuming the 35mA current rating is for the (undentified) sensor active electronics, regardless of the voltage (3.3V or 5V), any kind of decent cable will provide no significant voltage drop.  And there will be no significant current flow in the signal wire because of the (assumed) very high impedance of the destination (a microcontroller or ADC?)  

And even if there were some voltage drop, it is unlikely that would significantly affect the operation of the overall system. You could conceivably adjust for whatever detected offset it might cause.

If you want a more solid answer, we need to know:


What is the sensor?  Link to the specifications page.
Wnat is the destination of the signal?  Microconroller?  ADC?
What kind of wire (specifically the wire gauge) are you proposing?
What kind of calibration procedure does your system use for the sensor?