I think is no problem to use other cables for both hotend and thermistor. You can solder them and use shrink tube to isolate the solderings.
The currents are not so high, so you can just use other cables that have the same diameter of copper inside. Both cables have no special polarity, i would use red one for hotend and blue for the thermistor to make it easier when connecting to the board.
If you have connectors on the end, just cut the cable somewhere and solder a new cable between the cut.
liebe Grüße
Minihawk
*** In Wirklichkeit ist die Realität ganz anders ***
