You really just need to do the same thing that has been done for UART0 and other pad config definitions within bsp_padcfg.h. Anyway, as it turns out, our BSP uses UART3 on the same pin for RX anyway:
In bsp_padcfg.h:
#define UART3_PADS \
PAD_ENTRY(MII1_RXD3, (MODE(1) | PULLUDEN | RXACTIVE)) /* UART3_RXD */ \
PAD_ENTRY(MII1_RXD2, (MODE(1) | PULLUDEN)) /* UART3_TXD */ \
In basp_padcfg.c
const PAD_INFO UART3Pads[] = {UART3_PADS END_OF_PAD_ARRAY};
Then add
{ UART3Pads, AM_DEVICE_UART3},
entry to
const BOARD_PIN_MUX_T BOARD_PIN_MUX[] = {