Using the TX/RX LEDs as extra pins on the Pro Micro

ProMicroTXRXLEDRemap

The TX/RX LEDs on the Pro Micro uses the pins PD5 and PB0.
You can remove the resistors and use these pins, but you will need to first redefine the TXLED/RXLED pins.

In OS X:
/Users/USERNAME/Library/Arduino15/packages/SparkFun/hardware/avr/1.1.3/variants/promicro/pins_arduino.h

Change these values:
#define TX_RX_LED_INIT  DDRD |= (1<<5), DDRB |= (1<<0)
#define TXLED0          PORTD |= (1<<5)
#define TXLED1          PORTD &= ~(1<<5)
#define RXLED0          PORTB |= (1<<0)
#define RXLED1          PORTB &= ~(1<<0)

To:
#define TX_RX_LED_INIT  DDRD |= (1<<6), DDRF |= (1<<0)
#define TXLED0          PORTD |= (1<<6)
#define TXLED1          PORTD &= ~(1<<6)
#define RXLED0          PORTF |= (1<<0)
#define RXLED1          PORTF &= ~(1<<0)

* Now using PD6 for TX and PF0 for RX, but it can be any other pin not being used

Or:
#define TX_RX_LED_INIT  DDRD |= (1<<5), DDRB |= (1<<0)
#define TXLED0          1
#define TXLED1          1
#define RXLED0          1
#define RXLED1          1