Looks like you do need that resistor
Here is the quote on how to make that cable:
You can also check with Microsoft
The above diagram indicates that Pin #5 on the PC serial port should be connected directly to pin 4 on the UPS side, pin 8 to pin 2, pin 4 to pin 1, and pin 1 to pin 5. In addition, pins 5 and 8 on the same side (the UPS side) have a 10k ohm (10,000 ohm) resistor connected between them. This resistor serves to pull the voltage on this non-RS-232 output high until an actual low voltage is generated by a low battery condition
And putting a resistor inside of the cable is not a problem.