Heater cartridge extension wiring
Heater polarity doesn't matter
The heater cartridges are just large resistors and so polarity is irrelevant. Either can be positive or negative.
You can extend the leads by cutting and splicing in ~20 gauge wires* to a two pin JST connector line you suggest.

*At 24 volts and 30 watts, you need wire that is rated to carry at least 1.25 amps. The US National Electric Code dictates that this should be 20 gauge wire, but their standard is very conservative. Since you don't need to adhere to NEC codes, you could get away with something thinner (ie higher gauge number).